Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
In 10-inch skillet, cook bacon and onion over medium-high heat about 10 minutes, stirring occasionally, until bacon is crisp and onion is tender; drain on paper towels.
In large bowl, toss bacon mixture, potatoes and cheeses. Spoon into baking dish. In same bowl, beat eggs, milk, sour cream, salt, mustard and pepper with fork or wire whisk until well blended. Pour over potato mixture.
Bake uncovered 35 minutes. In small bowl, toss cornflake crumbs and butter. Sprinkle over casserole. Bake 10 to 15 minutes longer or until knife inserted in center comes out clean.