Clean fish, make 3 or 5 large slashes across each side of the fish and rub butter into slashes. Rub cavity with butter, salt, and pepper. Stuff with 1/2 of the lemonslices and oning. Lay out a layer of onion and lemon slices the size of the fish. Place the fish on them. Cover top with the remainder. Seal tightly in foil pouch, leaving a small opening at head. Pour beer into the opening and seal. Bake 45 minutes to 1 hour at 325 F. Serve with rice