Meanwhile, beat egg whites in medium bowl with mixer on high speed until stiff peaks form. Spray 2-qt. casserole with cooking spray; sprinkle 1 Tbsp. Parmesan evenly onto bottom and up side of casserole.
Place potatoes in large bowl; beat with mixer until smooth. Stir in shredded cheese, sour cream, milk, egg yolks, remaining Parmesan and half the onions. Gently stir in egg whites; spoon into prepared casserole.
Bake 40 to 45 min. or center is set and top is golden brown. Sprinkle with remaining onions.