Heat oil in large skillet on medium-high heat. Add onions; cook 5 min., stirring occasionally. Stir in zucchini; cook 3 min. Stir in peppers.
Top vegetables with ham; cover. Cook 1 to 2 min. or until heated through. Top with Singles. Remove skillet from heat. Let stand, covered, 30 sec. or until Singles are melted.
Fill rolls with ham and vegetable mixture.