Melt butter over medium heat in a small saucepan. Whisk in flour until well-combined, then whisk in milk, salt, pepper, cayenne pepper and 1 cup of Monterrey Jack cheese. Reduce heat to simmer and continue to whisk until sauce simmers slightly around the edges and has thickened slightly. Stir in remaining cheese.
Preheat oven to 350º F. Spray 9x13 casserole dish with nonstick cooking spray and set aside. Place potato slices in a bowl of cold water to keep from turning brown while making cream sauce.