Au Gratin Potatoes

This is a recipe I've recently found on Allrecipes that I love. For all those holiday and family gatherings when you're asked to bring a potato dish, this one gets rave reviews and you never have leftovers.

4 Russet potatoes, sliced into 1/4 inch slices
1 Onion sliced into rings
Salt and Pepper to taste
3 Tablespoons butter
3 Tablespoons all-purpose flour
1/2 Teaspoon salt
2 Cups Milk
1.5 Cups of Shredded cheddar cheese

Preheat oven to 400F (200C). Grease a 1 quart casserole dish. Layer the bottom with 1/2 the potatoes. Place all the onion rings on top of the potatoes and cover with remaining potatoes.
In a medium-size sauce pan, over medium heat, melt butter. Whisk in flour and salt and stir for one minute. Stir in milk and cook until mixture is thick. Stir in 1 Cup of cheese until fully melted. Pour mixture over potatoes. Sprinkle remaining shredded cheese over top and cover with aluminum foil.
Bake in oven 1.5 hrs or until potatoes are soft.