Preheat oven to 350° F. Spray 9×13 casserole dish with nonstick cooking spray and set aside.
Melt butter in a medium saucepan set over medium heat. Whisk in flour until well-combined, then whisk in milk, salt, pepper, cayenne pepper.
Reduce heat to simmer and continue to whisk until sauce bubbles slightly around the edges of the saucepan and has thickened slightly. If you are adding cheese, stir them in at this point.
Drain cold water from the potato slices and arrange the slices in the 9×13 casserole dish. Pour cream sauce over potato slices and cover the casserole dish with foil.