1. In a large bowl, toss trout, ¼ cup fresh herbs, lemon zest, red pepper, salt, black pepper, and ¼ cup olive oil. Let sit at room temperature, 20 minutes.
2. Heat oven to 350°. Drizzle both sides of bread with 1 tbsp olive oil, and season with salt to taste. Toast bread on a baking sheet until golden, flipping once, 8–10 minutes. Let cool, then crumble into small pieces.
3. In another bowl, whisk egg yolk, garlic, and 1 tsp lemon juice. Gradually drizzle in remaining ⅓ cup and 2 tbsp olive oil, whisking until dressing is thickened and emulsified, about 1 minute.
4. Add kohlrabies and remaining 1 tbsp lemon juice to trout mixture, and toss to combine. Divide dressing, kohlrabi salad, croutons, remaining ¼ cup fresh herbs, and Parmesan among four plates (or combine all in one large serving dish).