- 4 ounces smoked trout, broken into bite-size pieces
- 1 teaspoon finely grated lemon zest
- 1/4 teaspoon crushed red pepper flakes
- 1/2 cup finely chopped mixed tender herbs (such as parsley, chives, and/or fennel fronds), divided
- 1/4 cup plus 2 tablespoons olive oil; plus more for drizzling
- Kosher salt, freshly ground pepper
- 4 slices (1/4-inch-thick) country-style bread
- 1 large egg yolk
- 1 garlic clove, finely grated
- 1 teaspoon plus 1 tablespoon fresh lemon juice
- 1/3 cup vegetable oil
- 2 pounds kohlrabies (about 5 medium), peeled, halved lengthwise, very thinly sliced
- Shaved Parmesan (for serving)
- Gently toss smoked trout, lemon zest, red pepper flakes, 1/4 cup mixed herbs, and 1/4 cup olive oil in a large bowl to combine; season with salt and black pepper. Let sit at room temperature 20 minutes.
- Meanwhile, preheat oven to 350°F. Drizzle both sides of bread with olive oil; season with salt. Toast on a rimmed baking sheet until golden brown and crisp, 8–10 minutes. Let cool, then crumble into small pieces. Set croutons aside.
- Whisk egg yolk, garlic, and 1 tsp. lemon juice in a medium bowl to combine. Whisking constantly, gradually add vegetable oil and 2 Tbsp. olive oil, drop by drop at first, then in a steady drizzle; whisk until aioli is thickened and emulsified. Stir in 1 tsp. water to loosen sauce; season with salt.
- Add kohlrabies and remaining 1 Tbsp. lemon juice to bowl with smoked trout mixture. Season with salt and pepper, drizzle with olive oil, and gently toss to combine.
- Spread aioli on a large plate and top with kohlrabi salad, shaved Parmesan, reserved croutons, and remaining 1/4 cup mixed herbs.
- Aioli can be made and trout marinated 1 day ahead. Cover and chill separately.