Preheat oven to 400°F. Lightly spray an 8×8-inch baking dish.
Arrange potato and onion slices in the dish, alternating layers.
Melt butter in saucepan, stir in flour and salt. Cook 1 minute.
Gradually whisk in milk until smooth and thickened.
Stir in shredded cheddar until melted and smooth.
Pour cheese sauce over potatoes. Cover with foil.
Bake 85–90 minutes, until tender.
Broil uncovered for 2–3 minutes to crisp top.
Season with salt and pepper, let rest 5 minutes, then enjoy.
