Preparation - In a small skillet, saute the bell pepper in 1 tablespoon butter or margarine for about 5 minutes or until barely tender. Add the milk and cheese and cook over low heat until the cheese is melted. Remove from heat. Stir in the diced tomato; cover and keep warm. In a mixing bowl, combine the lightly beaten eggs with the water and salt. Stir in the instant mashed potato flakes. Melt 3 tablespoons butter in a large skillet. Add the egg mixture and cook over medium heat unitil the bottom begins to set, lifting the edges of the omelet and tilting the pan, allowing the uncooked egg mixture to reach the bottom of the pan. Cook until the bottom is very lightly browned and the top is set. Spoon half of the sauce on the omelet; fold the omelet in half. Turn onto a serving plate and drizzle the remaining cheese sauce over top.