Kohlrabi Caesar Salad

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.