Preheat oven to 450ºF. Line a 9-by-13-inch baking pan with foil and mist with cooking spray.
Pat fish dry with paper towels. Sprinkle with pepper; transfer to pan. Combine crushed crackers and dill in a bowl. Mix mayonnaise, mustard and lemon juice in another bowl.
Spread mayonnaise mixture over tops of fillets. Sprinkle cracker mixture on top and gently press to adhere.
Bake until fish is cooked through, 10 to 15 minutes.