Preheat the oven to 350° Fahrenheit.

Lightly coat a 9x13-inch casserole dish with butter or non-stick cooking spray.

Evenly line the casserole dish with the sliced potatoes.

Set aside while making cream sauce.

For cream sauce, melt the butter in a small saucepan over medium heat.

Slowly add the flour to the melted butter and whisk until well combined.

Stir in the milk, salt and pepper.

Reduce heat to a simmer and add the cheeses to the saucepan, stirring until fully melted and the sauce thickens.

Pour the cream sauce over potato slices in the casserole dish.

Place uncovered in the preheated oven and bake for one hour.