Thinly slice potatoes into rounds and place in a large bowl. Cover with water to prevent browning and set aside.
In a large sauce pan over med. high heat, completely melt butter. Slowly stir in flour until you have a thick paste. Allow to heat until bubbly. Add milk, heavy cream, salt, pepper, and onion powder. Add 3/4 of cheese cubes. Stir until almost bubbly and all ingredients are fully incorporated into paste. Remove from heat.
Drain potatoes and line bottom of baking dish with a layer of potatoes, letting the potatoes slightly overlap. Top with 1/3 of cheese sauce. Repeat layering until all of sauce and potatoes are used. Top with the remaining cheese and cover with aluminum foil.
Bake in preheated oven for 45 minutes. Remove foil and turn heat to 400 degrees F. Bake for an additional 15 minutes. Cheese will be slightly browned and bubbly. Remove from oven and let sit for 5 minutes before serving.