Rinse fillets and pat dry. Slice onion into 1/8-inch rings, and set aside. Slice lemon into 1/8-inch rings, remove seeds and set aside.
Rub 1 clove of crushed garlic over the fillet. Place fillet in the center of the aluminum foil. Top with 1 pat of butter, 2 to 3 onion rings, then season with salt and pepper, to taste. Cover fish with 1 or 2 lemon rings. Fold aluminum foil around fish to create a sealed package. Repeat for each fillet.
Place packets on hot grill for 5 minutes or until cooked through, careful not to overcook the fillets. Remove fish from foil, plate and serve immediately.