On a large sheet of foil, place the green beans, red potatoes, onion, and sausage. Season with salt and pepper, sprinkle with oil, and top with butter. Tightly seal foil around the ingredients, leaving only a small opening. Pour water into the opening, and seal.
Place foil packet on the prepared grill. Cook 20 to 30 minutes, turning once, until sausage is browned and vegetables are tender.
