Place mahi mahi in an aluminum foil pan, and toss with the garlic to coat. Distribute butter evenly throughout pan. Spread onions over fish. Pour the lemon juice, wine, and diced tomatoes with green chile peppers over the fish. Season with salt and pepper. Tightly cover pan with aluminum foil.
Place pan on the grill grate, and cook fish 35 minutes, or until easily flaked with a fork. Sprinkle with cheese before serving.