HEAT oven to 375°F. Cook onions in dressing in large nonstick skillet on medium heat 8 to 10 min. or until golden brown, stirring frequently. Remove from heat; cool slightly.
MIX meat, onions, 1/4 cup ketchup, stuffing mix, water and egg whites. Shape into 10x5-inch oval loaf in 13x9-inch baking dish; cover with remaining ketchup.
BAKE 55 min. to 1 hour or until cooked through (160°F). Sprinkle with cheese; bake 2 min. or until melted.