HEAT oven to 375ºF.
COOK onions in butter in large nonstick skillet on medium heat 8 min. or until tender, stirring frequently. Blend in flour; cook and stir 1 min. Gradually stir in broth; cook 5 min. or until thickened, stirring constantly. Add 1 cup shredded cheese and pepper; cook and stir 1 min. or until cheese is melted. Remove from heat.
LAYER potatoes alternately with cheese sauce in 2-qt. casserole sprayed with cooking spray, ending with sauce. Top with remaining shredded cheese and Parmesan; cover.
BAKE 1-1/2 hours or until potatoes are tender and top is golden brown, uncovering for the last 15 min. Top with onions.