Creamy Au Gratin Potatoes

4 potatoes, sliced into 1/4 in slices
1 onion, chopped
salt and pepper to taste
3 Tbs. butter
3 Tbs. flour
1/2 tsp. salt
1 garlic clove, minced
2 c. milk
1 1/2 c. shredded cheddar cheese

Preheat oven to 400 degrees. Grease a 1 quart casserole dish. Layer 1/2 of the potatoes into the casserole dish top with onions, and add remaining potatoes. Season with salt and pepper to taste. In a medium sized saucepan, melt butter over medium heat. Mix in the flour and salt, stir constantly with a whisk for one minute. Stir in milk and garlic, cook until mixture has thickened. Stir in cheese all at once, and continue stirring until melted, about one minute. Pour cheese sauce over potatoes, and cover with aluminum foil. Bake 1 1/2 hours.