HEAT oven to 350ºF.
HEAT oil in large skillet on medium heat. Add onions; cook 10 min., stirring occasionally. Add peppers; cook 5 to 6 min. or until onions are golden brown and onions and peppers are tender. Remove from heat.
ADD dressing and mustard to onion mixture; mix well. Stir in mozzarella; spread onto bottom of 9-inch pie plate. Top with Parmesan.
BAKE 18 to 20 min. or until heated through.