Chilled Watercress and Kohlrabi Soup with Smoked Trout
Kohlrabi simmered soft with shallot and blended with raw watercress into a cold pale green soup, finished with flaked smoked trout, dill, and lemon zest.
Why You'll Love This Recipe
Every soup I made in the summer used to be one I was too warm to eat, which is how this one came about. It goes into a jar cold, travels to work with no reheating at all, and tastes like something you chose rather than something you settled for in August. The silky texture comes from kohlrabi cooked until it is completely soft and then blended with good olive oil, which is a trick worth knowing for any cold soup. Twenty minutes on the stove and two hours in the refrigerator is the whole commitment, and the trout on top needs nothing done to it beyond a fork and a minute of your attention.
The order here decides the color. Kohlrabi simmers in broth with shallot and garlic until it gives with no resistance, and then the pot comes off the heat and cools before anything green goes near it. The watercress goes into the blender raw, at the very end, and thirty seconds of blending is plenty. Hot watercress turns olive drab and slightly bitter within a couple of minutes, and once it has gone that way there is no bringing it back. Pour the last of the olive oil in while the blades are running so it emulsifies into the soup, and that is where the smooth, faintly rich body comes from.
Cold food tastes less salty than warm food does, so taste this again once it has chilled and expect to want a pinch more than felt right at the blender. The trout is what turns it from a first course into lunch. Hot-smoked fillets flake into large oily pieces that sit on the surface rather than sinking, so every few spoonfuls you get cold peppery green soup and then a mouthful of smoky fish. Dill, chives, and lemon zest over the top do the rest of the work, and a ribbon of olive oil across the surface catches the light. On a day when nothing hot appeals, this is the bowl I want in front of me.
Ingredients
- 2 kohlrabi bulbs (about 1 1/4 lb), peeled and cut into 1/2-inch cubes
- 2 bunches watercress (about 5 oz), thick stems trimmed
- 8 oz hot-smoked trout fillets, skin removed and flaked
- 2 large shallots, thinly sliced
- 1 clove garlic, thinly sliced
- 3 1/2 cups low-sodium vegetable broth
- 3 tbsp olive oil, divided
- 2 tbsp fresh lemon juice, plus the zest of 1 lemon
- 2 tbsp chopped fresh dill
- 3 tbsp chopped fresh chives
- 3/4 tsp fine sea salt, divided
Instructions
- Soften the shallot: Heat 2 tablespoons of the olive oil in a saucepan over medium heat. Add the shallots, the garlic, and a quarter teaspoon of the salt, and cook for 4 to 5 minutes, stirring, until they are soft and translucent. Keep them pale, since any browning shows up as a muddy note in a cold soup.
- Simmer the kohlrabi: Add the kohlrabi cubes and the vegetable broth and bring the pan to a simmer. Cover and cook for 15 to 18 minutes, until a cube crushes easily against the side of the pan with a spoon. Kohlrabi is dense, and undercooked pieces are the main reason a blended soup comes out grainy.
- Cool the base: Take the pan off the heat and pour the contents into a wide bowl. Let it sit for 15 to 20 minutes, or set the bowl in an ice bath and stir for 5, until the soup is no more than warm. Anything hotter will cook the watercress in the blender and dull the color.
- Blend with the watercress: Put the cooled base and the trimmed watercress in a blender and run it for 30 to 45 seconds, until it is smooth and bright green, pouring in the remaining tablespoon of olive oil as it goes. Add the lemon juice and the remaining salt, blend for 5 seconds more, and taste it.
- Chill: Refrigerate the soup for at least 2 hours, until it is properly cold. Taste it again once it is chilled and add another pinch of salt or a squeeze of lemon if it has gone flat, which cold food usually does. If you would rather eat it barely warm, warm it gently over low heat and stop well before it simmers.
- Flake the trout and serve: Pull the skin off the trout fillets and flake the flesh into large pieces with a fork, checking for pin bones as you go. Ladle the soup into bowls, pile the trout in the middle, and finish with the dill, the chives, the lemon zest, and a thin ribbon of olive oil.
Variations
- Smoked salmon: Lay ribbons of smoked salmon over the top in place of the trout for a softer, silkier piece of fish against the peppery soup.
- Sauerkraut on top: Spoon a tablespoon of drained sauerkraut onto each bowl for a sharp, crunchy bite that cuts through a smooth cold soup.
- Richer base: Use chicken bone broth in place of the vegetable broth when you want the soup rounder and more savory, especially if you are serving it warm.
- Herb heavy: Blend a handful of parsley and mint in with the watercress for a greener, more garden-tasting bowl.
- Served warm: Warm it gently and serve it in mugs with the trout on the side, which is how I like it once the weather turns.
Storage & Leftovers
The soup keeps in a sealed jar in the refrigerator for 3 days, and the trout keeps for 3 as well, stored separately so it does not soften in the liquid. The green fades toward olive after about a day and a half, which affects how it looks rather than how it tastes.
There is nothing to reheat here, and that is the point of it. Take it out of the refrigerator, stir it, and eat it cold. If you want it barely warm on a cooler day, heat it gently in a saucepan over low and pull it off before it simmers, since boiling a blended watercress soup turns it drab and slightly bitter.
Freezing does not work for this one. The blended kohlrabi separates on thawing and comes back watery and grainy, and the raw watercress loses everything that made it worth blending in. If you want a head start, cook and chill the kohlrabi base up to 2 days ahead and blend the watercress in on the day you want it.
Notes & Tips
- Take the kohlrabi skin off with a knife: There is a tough fibrous layer just under the peel that never softens, and a vegetable peeler leaves most of it behind. A paring knife takes it off cleanly.
- Cook the kohlrabi all the way: A cube should collapse under a spoon before you blend. Anything firmer leaves a fine grain in the soup that no amount of blending fixes.
- Cool the base before the greens go in: This is the step that keeps the soup bright green. Hot liquid cooks the watercress in the blender and the color goes within a minute or two.
- Blend the oil in at the end: Pouring the last tablespoon in while the blender runs emulsifies it, and that is where the smooth, faintly rich texture comes from.
- Season it twice: Cold dulls salt and acid, so taste once at the blender and again after chilling. The second taste is the one that matters.
- Buy hot-smoked trout, not cold-smoked: Hot-smoked fillets are firm and flake into big pieces. Cold-smoked trout is silky and translucent and will disappear into the soup instead of sitting on it.
Nutrition & Mindful Eating
- Watercress, kohlrabi, and smoked trout each carry a different part of this bowl. The kohlrabi gives the soup its body, the watercress gives it color and pepper, and the trout on top brings the protein and the fat that make a cold soup eat like a full lunch.
- Watercress is one of the most concentrated leaves in the produce aisle, and blending it in raw keeps its vitamin C and vitamin K intact rather than cooking them down. That sharp, peppery taste comes from the same family of compounds you find in mustard and horseradish.
- Kohlrabi is a cabbage-family vegetable that grows as a bulb above ground, and cooked soft it blends to a texture close to a cream soup. It carries vitamin C and a good amount of fiber, which is what keeps a smooth bowl from feeling insubstantial.
- Smoked trout is an oily fish, and it brings omega-3 fats and vitamin D along with the protein that anchors the meal. Hot-smoked fillets flake into big pieces, so you taste them clearly rather than losing them in the soup.
- Lemon juice goes in at the blender and the zest goes on at the last second, since the zest holds the oils that fade fastest. With dill and chives over the top, the finished bowl tastes green and sharp before it tastes of anything else.
- As you eat, notice how cold the first spoonful is against the warm smokiness of the trout, and let a lunch you did not have to heat still be one you sit down with for a while.
Frequently Asked Questions
What if I cannot find watercress?
Arugula is the closest thing in most stores and gives you a similar pepper with a slightly softer green color. Baby spinach works too and makes a milder, sweeter soup that will want an extra squeeze of lemon. Use the same weight in every case and add it raw, at the end.
Can I use a different fish?
Yes. Smoked salmon is the easy swap and sits on top just as well, sliced into ribbons rather than flaked. Smoked mackerel is stronger and oilier, and canned sardines drained and broken up work perfectly well if you like them. All three go on cold, with nothing cooked.
Is this supposed to be thick or thin?
Somewhere in between, thick enough to coat the back of a spoon and thin enough to drink from a mug. It sets up noticeably in the refrigerator, so if it is stiffer than you want after chilling, stir in a few tablespoons of cold broth or water before serving.
Does this fit an AIP elimination phase?
It does, with one thing to check. There are no grains, legumes, dairy, eggs, nuts, seeds, or nightshades here, but smoked fish is often cured with sugar, pepper, or a spice rub, so read the package and buy trout smoked with salt and smoke alone. Check the broth carton for pepper too.