Scrub and pierce potatoes; place on a microwave-safe plate. Microwave on high for 15-20 minutes or until tender. Cool slightly. In a saucepan, melt butter. Stir in the flour, garlic powder, salt and pepper until smooth; gradually whisk in milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Add the process cheese and bacon; stir until cheese is melted. Remove from the heat; set aside. Cut potatoes into 1/4-in. slices. Place a third of the slices in a greased 13-in. x 9-in. baking dish; top with a third of the cheese sauce. Repeat layers twice. Sprinkle with cheddar cheese and onions. Bake, uncovered, at 350° for 15 minutes or until cheese is melted.