HEAT oven to 400ºF.
COOK first 3 ingredients in saucepan on medium heat until cream cheese spread is melted and mixture comes to boil, stirring constantly with whisk.
RESERVE 2 Tbsp. bacon. Layer half each of the potatoes, onions and remaining bacon in 13x9-inch baking dish; repeat layers. Add cream cheese sauce; cover.
BAKE 1-1/2 hours or until potatoes are tender and top is golden brown, uncovering and topping with shredded cheese and reserved bacon for the last 10 min.