Microwave Singles and milk in microwaveable bowl on HIGH 50 sec. or until blended, stirring every 30 sec. Stir in mustard.
Heat heavy medium skillet sprayed with cooking spray on medium heat. Add eggs, 1 at a time, allowing space between eggs. Pour water around eggs; cover. Bring just to gentle boil; simmer 2 min. or until whites are completely set and yolks are cooked to desired doneness.
Cover muffin halves with ham, spinach and eggs; top with sauce, onions and tomatoes.