Rinse fish and pat dry. Season to taste. Lay fish, skin side down, on a piece of heavy-duty foil. Cut foil to follow outline of fish, leaving a 1- to 2-inch border. Crimp edges of foil to fit against fish. Arrange foil-supported fish on cooking grate. Place lid on grill. Cook until fish is opaque but still moist (about 10 minutes).
Supporting fish with foil and wide metal spatula, transfer to a platter. Season to taste.