Prepare the Sauce:
In a medium saucepan, melt the butter over medium heat. Next, add the flour, salt, and pepper and whisk constantly.

Once the mixture has thickened a little, gradually add the milk and whisk thoroughly before adding more milk.

Continue whisking until the mixture is fully thickened.

Now, it’s time to remove the pan from the stove and stir in the shredded cheese. Stir until combined and serve.

How to Assemble Your Scalloped Potatoes
Thinly slice your leftover potatoes. As mentioned earlier, if you're boiling your potatoes specifically to make this recipe, allow them to cool first or you’ll burn your fingers and end up with crumbly potatoes.

If you are using leftover cheese sauce, you don’t need to reheat the cheese sauce before using it. Simply drop it by spoonfuls on the potatoes and the heat from the oven will help distribute the sauce evenly.

Place half the potatoes in an oven-safe dish. Top with half the cheese sauce. Then add another layer of potatoes. Finally top with the remaining cheese.

Place the baking dish, uncovered, into a 350F oven and bake for 20-25 minutes. In the last 10 minutes of baking, top with some shredded cheese and allow it to brown.