In a medium bowl, mix together 1/2 cup Parmesan cheese, 1/4 cup softened butter, 4 tablespoons mayonnaise, 2 tablespoons lemon juice, 1/4 teaspoon dried parsley, 1/4 teaspoon ground black pepper, 1/4 teaspoon garlic salt, 1/8 teaspoon onion powder, and 1/8 teaspoon celery salt until well combined and spreadable.
Pat 2 pounds tilapia fillets dry with paper towels and place them in a single layer on the prepared baking sheet.
Spread the Parmesan mixture evenly over the top of each fillet.
Bake on the center rack at 350°F (175°C) for 15–20 minutes, or until the topping is light golden brown and the fish flakes easily with a fork.