In a 1 quart casserole, combine the corn, green or sweet red pepper, onion and water. Cook, covered on 100% power (high) for 6 to 8  minutes (10 to 12 minutes on low wattage ovens) or until the corn and onion are tender, stirring once. Stir in the margarine, mustard, salt and pepper. Cook, covered, on high for 1 minute more. Stir  to mix well.