Preheat oven to 400 degrees F (200 degrees C). Place butter in a 9x13-inch casserole dish.

Place casserole dish in the preheating oven until butter is melted, about 5 minutes.

Spread onion slices in the bottom of the casserole dish. Place cod skin-side down over onion. Sprinkle seasoned salt evenly on top.

Mix mayonnaise, Parmesan cheese, provolone cheese, mozzarella cheese, and fresh parsley together in a bowl. Spread over cod.

Bake in the preheated oven until cod flakes easily with a fork, 20 to 25 minutes.