Pre-heat grill to medium high heat. Rinse fillets and pat dry. Slice onion into 1/8 inch rings. Set aside. Slice lemon into 1/8 inch rings. Remove seeds and set aside. Place each fillet in center of an approximate 12 x 12 inch piece of heavy duty aluminum foil. Top each fillet with garlic, 1 pat of butter, 2 to 3 onion rings. Season with salt and pepper. Cover fish with 2 lemon rings. Fold aluminum foil around fish to create a sealed package. Repeat for each fillet. Place fish packets on grill for 5 - 7 minutes or until cooked thoroughly. Remove from grill and serve.
Fish should be cooked for 8 – 10 minutes per inch of thickness measured at the thickest point. To test, use the tip of a small knife and make a small slit in the fish. The flesh should flake easily and the center should appear opaque.
To eliminate any fishy smells on your hands, rub your hands with a lemon wedge, then rinse under cold water.