Preheat oven to 400°F. Lightly grease an 8×8-inch baking dish.
Arrange potato slices evenly in the dish.
In a medium saucepan, melt butter over medium heat. Whisk in flour and salt; cook for 1 minute.
Slowly whisk in milk. Cook until thickened, about 2–3 minutes.
Stir in cheddar cheese and mix until smooth.
Pour cheese sauce over the potatoes. Cover tightly with foil.
Bake for 85–90 minutes, until the potatoes are fork-tender.
Broil 2–3 minutes for a golden top.
Season with salt and pepper to taste.