1Heat oven to 375°F. Lightly spray 1 1/2- to 2-quart casserole with cooking spray.
2In 12-inch skillet, melt butter over medium heat. Cook onions in butter 30 to 40 minutes, stirring often, until caramel colored.
3In large bowl, mix cheeses, mayonnaise and onions. Spoon into casserole.
4Bake uncovered 30 minutes or until golden and bubbly. Serve with sweet potato chips.