4 cups potatoes, sliced very thin (I scrub the potatoes with a brush and do not peel. You can peel if you prefer)
3 tablespoons butter
3 tablespoons flour
1 1/2 cups milk plus a splash more if you like them extra creamy
1 small onion, finely chopped
1 teaspoon salt
1/4 tsp pepper
1 1/4 cup grated sharp cheddar cheese, divided
paprika, if desired
Melt butter in a small pan; add onion and cook several minutes. Stir in flour; cook 2 minutes.
Whisk in milk, stirring constantly; bring to a simmer and cook 2 more minutes.
Remove from heat and add salt, pepper and a few swipes across a micro-plane of whole nutmeg. Stir in 1 cup cheese and ketchup, if using.
Place a half of the sliced potatoes in a lightly buttered or sprayed one-quart casserole dish.
Pour half of cheese sauce over potatoes.
Repeat with second layer of potatoes and cheese sauce.
Sprinkle the remaining cheese. Sprinkle lightly with paprika.
Bake uncovered for about 1 hour at 350°F. (I bake in toaster oven and cover with foil if browning too much).