1.
Preheat an outdoor grill for Medium-High heat.
2.
Clean and pat dry fish, season with salt and pepper, and rub both sides of fish with butter. Fill cavity with lemon and onion slices, and season with salt and pepper.
3.
Place fish on grill, top with more lemon and onion slices, tomato slices, and salt and pepper to taste.
4.
Grill for 20 minutes, turn, and grill until fish flakes easily with a fork, about 20 minutes.