Scalloped Potatoes

4  potatoes (1lb 8oz to 2 lbs), sliced thinly
1 small (4oz) onion, sliced
3  Tbsps (42gr) butter
3  Tbsps flour
1½  cups milk
1  tsp salt
1  dash cayenne pepper
1½  cups (105gr) cheddar cheese, sharp, grated
1  dash paprika

1. Place oven rack in the middle position. Pre-heat oven to 350°F.
2. In a small sauce pan, melt butter and blend in flour. Let sit for a minute. Add all the cold milk, stirring with a whisk and season with salt and cayenne. Cook at a low temperature until smooth and boiling, stirring occasionally with the whisk. Remove from heat and stir in the cheese.
3. Place a quarter of the sliced potatoes in a lightly greased one quart casserole dish. Layer with half the onion. Top with another quarter of potatoes, then pour in half of cheese sauce as evenly as possible.
4. Repeat with a third layer of potatoes, the rest of the onion, and the final portion of potato, topped with the rest of the cheese sauce.
5. Sprinkle the remaining cheese on top, colour with a dash of paprika.
6. Bake uncovered for about 1 hour or until the potatoes are soft.