Heat grill to medium heat.
Grill wieners 7 to 9 min. or until heated through, turning occasionally.
Meanwhile, heat oil in medium skillet on medium heat. Add onions and peppers; cook and stir 5 min. or until crisp-tender. Add next 3 ingredients; cook 3 min. or until most of the liquid is absorbed. Remove from heat.
Spread cut sides of buns with mayo, then mustard. Fill with Singles, tomatoes, wieners and onion mixture.