Clean the fish and leave the heads on. Spray the aluminum foil with Pam. Place fish on foil. Lay 2 or 3 onion slices and 1 lemon slice on each fish. Fold the aluminum foil and place on the campfire. It works best if you heat a flat rock in the middle of the fire. It doesn't take long for the fish to cook. Check it every now and then. The fish will appear white and flaky when done.