STEP-1
Preheat the oven to 350°F. Spray a 9 x 9 baking dish with cooking spray and set aside. In a medium saucepan, melt the butter. Whisk in the flour, and cook 3-4 minutes, until golden brown.
STEP-2
Slowly add the milk and cream, whisking constantly to prevent lumps. Cook an additional 5-6 minutes, or until thickened. Remove from heat and stir in the salt and pepper and one cup of the cheddar cheese.
STEP-3
Layer one-third of the potato slices as evenly as possible in the prepared baking dish. Pour one-third of the sauce over the potatoes and spread to cover. Sprinkle ⅓ of the parmesan cheese and ⅓ of the remaining cheddar cheese over the top. Repeat the layers twice more.
STEP-4
Cover with foil and bake for one hour. Remove foil and bake for an additional 20-30 minutes, or until golden brown and bubbling on top.
STEP-5
Let cool 15-20 minutes before serving.