Heat oven to 350 degrees and arrange a rack 8 inches from the top broiler.

Tear the bread into rough 1-inch chunks and add them to a large baking sheet. Sprinkle with 1 teaspoon garlic powder, season with salt and a good amount of pepper, and drizzle with the olive oil. Toss everything to coat, then spread bread evenly on the pan. Bake for 10 to 12 minutes, tossing halfway through, or until bread is golden brown on the outside but still slightly chewy in the center. Remove the croutons from the oven, dump them into a large bowl, and wipe the crumbs from the sheet pan.

Turn on the broiler. To a medium bowl, add the mayonnaise, lemon juice, Dijon mustard, Worcestershire, fish sauce, the remaining 1 teaspoon garlic powder, and 1 teaspoon pepper, and whisk to combine, adjusting the seasoning, if desired. Remove ¼ cup dressing, and place it in a small mixing bowl along with ¼ cup grated Parmesan. Mix to combine.

Pat the salmon dry with a paper towel, and season lightly with salt and pepper. Using a pastry brush, liberally coat the fillets all over with the reserved Parmesan dressing. Place on the baking sheet, top fillets with the remaining ¼ cup Parmesan, and place under the broiler for 3 to 5 minutes, or until the salmon is browned on the surface and cooked through. If it starts to smoke, move the pan to a lower rack to finish cooking.

Remove the salmon from the oven, and allow a few minutes to cool slightly.

To a large bowl, add the lettuce, croutons and sliced onion. Drizzle in some of the dressing, and using tongs, toss to evenly coat the leaves. Divide across salad bowls and sprinkle with more Parmesan. Top each salad with a fillet and serve.