BEAT eggs and mustard with wire whisk until well blended; set aside.
MELT margarine in 8-inch skillet on medium heat.
ADD egg mixture and cheese. As egg mixture sets, lift slightly with spatula to allow uncooked portion to flow underneath. When egg mixture is set but top is still moist, add vegetables to half of omelet. Slip spatula underneath, tip skillet to loosen and gently fold omelet in half. Slide onto serving plate.