Adjust oven rack to middle position and heat oven to 350 degrees.

Pulse bread, melted butter, 1/4 teaspoon salt, and 1/4 teaspoon pepper in food processor until bread is coarsely ground, eight 1-second pulses (you should have about 3 1/2 cups crumbs).

Transfer to rimmed baking sheet and bake until deep golden brown and dry, about 15 minutes, stirring twice during baking time. Cool crumbs to room temperature, about 10 minutes.

Transfer crumbs to pie plate; toss with parsley and shallot. Increase oven temperature to 425 degrees.

Place 1/4 cup flour in second pie plate.

In third pie plate, whisk eggs, horseradish (if using), mayonnaise, paprika, cayenne pepper (if using), and 1/4 teaspoon black pepper until combined; whisk in remaining 5 tablespoons flour until smooth.

Spray wire rack with nonstick cooking spray and place in rimmed baking sheet.

Dry fish thoroughly with paper towels and season with salt and pepper.

Dredge 1 fillet in flour; shake off excess.

Using hands, coat with egg mixture. Coat all sides of fillet with bread crumb mixture, pressing gently so that thick layer of crumbs adheres to fish.

Transfer breaded fish to wire rack. Repeat with remaining 3 fillets.

Bake fish until instant-read thermometer inserted into centers of fillets registers 140 degrees, 18 to 25 minutes.

Using thin spatula, transfer fillets to individual plates and serve immediately with lemon wedges.