In a saucepan melt butter over low heat. Add flour and stir (using a wooden spoon). In a separate saucepan have hot milk over low to medium heat. While constantly whisking, gradually add hot milk to the roux (the butter and flour mixture). Simmer mixture while continuing to whisk. Sauce will thicken in a few minutes.
***To basic finished bechamel sauce, add 1/4 cup grated Gruyere and 1/4 cup grated Parmesan to sauce. Slice potatoes and layer in gratin dish. Season potatoes with salt and pepper to taste. Pour bechamel with cheese over potatoes and bake for 30 minutes in 350 degree oven, until potatoes are cooked through. Garnish with parsley