Step 1

Preheat oven to 350°F. In a small skillet over medium-low heat, melt butter. Add garlic and cook, stirring, until fragrant, 30 seconds. Add bread crumbs, stir to coat with butter, then cook, stirring frequently, until light golden brown, about 5 minutes. Remove from heat and stir in parsley and lemon zest. Let cool. Combine mustard and mayonnaise in a small bowl.

Step 2

Rinse fish and pat dry. Sprinkle each fillet lightly with salt, then place on a rimmed, foil-lined baking sheet and brush with mustard mixture. Press 1/4 of crumbs onto each filet.

Step 3

Transfer baking sheet to oven and bake until fish is no longer translucent (cut to test) and flakes easily, 10 to 15 minutes. If crumbs aren't sufficiently browned, broil fish 2 inches from heating element until crumbs are crisp and dark golden brown (watching carefully to prevent burning), 2 minutes longer. Serve immediately.