HEAT oven to 400ºF.
HEAT dressing and oil in large nonstick skillet on medium heat. Add onions; cook 10 to 15 min. or until onions are caramelized, stirring frequently. Set aside.
LAYER 1/3 each of the potatoes, onions and shredded cheese in 2.5-L casserole dish sprayed with cooking spray. Repeat layers twice; cover.
BAKE 30 min.; sprinkle with Parmesan. Bake, uncovered, 45 min. or until potatoes are tender and top is lightly browned. Let stand 5 min. before serving.
serving size = 1/2 cup (125 mL)