How to Make It
Preheat oven to 425°F. Toss together bread cubes, 1/4 cup olive oil, 3 smashed garlic cloves, 3/4 teaspoon salt, and 1/4 teaspoon pepper on a rimmed baking sheet until bread is evenly coated. Spread in a single layer.
Bake in preheated oven, stirring occasionally, until golden brown, 12 to 16 minutes. Remove croutons from oven, and let cool completely, about 15 minutes.
Meanwhile, combine canned trout (with liquid), about one-third of the Parmesan, egg yolk, lemon juice, capers, Dijon, Worcestershire sauce, remaining smashed garlic clove, remaining 3/4 teaspoon salt, and remaining 1/2 teaspoon pepper in a blender. Process until smooth, about 20 seconds. With blender running, add remaining 1/4 cup olive oil in a slow, steady stream, processing until smooth, about 20 seconds.
Toss together dressing, romaine, radicchio, croutons, and one-third of the Parmesan in a large bowl until coated. Transfer to a serving platter, and top with remaining Parmesan.