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