I've got the blues.

This recipe would work for other types of fish as well.
Rather than adding the second 2 tbsp of butter for basting, I just used a pasty brush and marinade in the dish while broiling.
BLUEFISH AU GRATIN

3/4 lb Bluefish fillets, cut in 2 pieces
4 tbsp melted butter, divided
2 tsp white Worcestershire sauce
1 tbsp lemon juice
Pinch of salt, oregano, thyme, dill weed and basil
2 medium onions, sliced
1/3 cup cheddar cheese, grated

Combine 2 tbsp melted butter, Worcestershire sauce, lemon juice, salt and spices. Pour over fillets in a glass baking dish and marinate for at least 1 hour in the refrigerator.
Sauté onions about 15 minutes or until tender and set aside.
Broil fish 4 inches from the broiler and baste with remaining butter for 10 to 12 minutes per inch of thickness. When fish is done, spoon onions over fillets and sprinkle with cheese. Return to broiler for 30 seconds to melt cheese. Serves 2.
Variation: Substitute 1/2 green pepper for 1 onion.

We served this with brown rice.