Combine the mayonnaise, butter, onion, Parmesan, lemon juice, zest, parsley and Old Bay. Season with salt and black pepper, to taste. Mix well.
Place the fish in a baking dish that’s been sprayed with nonstick cooking spray. Spread the mayonnaise mixture over the fish fillets. Sprinkle with the breadcrumbs.
Bake in a preheated 400-degree F oven for about 10 minutes. Turn the broiler on and cook until the top is golden brown.
