Creamy Au Gratin Potatoes

4 large russet potatoes, sliced into 1/4 inch slices
1medium onion, chopped
     salt and pepper to taste
3tablespoons butter
3tablespoons all-purpose flour
1/2teaspoon salt
2cups milk
1-1/2cups shredded Cheddar cheese

1. Preheat oven to 400 degrees F (200 degrees C). Butter a 1 quart casserole dish.
2. Layer 1/2 of the potatoes into bottom of the prepared casserole dish. Top with the onion, and add the remaining potatoes. Season with salt and pepper to taste.
3. In a medium-size saucepan, melt butter over medium heat. Mix in the flour and salt, and stir constantly with a whisk for one minute. Stir in milk. Cook until mixture has thickened. Stir in cheese all at once, and continue stirring until melted, about 30 to 60 seconds. Pour cheese over the potatoes, and cover the dish with aluminum foil.
4. Bake 1 1/2 hours in the preheated oven.
   
Note:  I microwaved the peeled potatoes for one minute prior to slicing... not sure that this made a difference.