Cook noodles as directed on package, omitting salt. Meanwhile, combine mashed potatoes, cream cheese spread, onions, 1/2 cup bacon and 1 cup Cheddar.
Place 3 noodles in 13x9-inch baking dish sprayed with cooking spray; cover with 1/3 of the potato mixture. Repeat layers twice. Top with remaining bacon and Cheddar; cover.
Bake 30 min. or until heated through. Uncover; bake 3 to 5 min. or until Cheddar is lightly browned.