Potatoes Au Gratin

8 cups thinly sliced potatoes (Yukon Gold or Russet)
1 onion (optional)
6 Tablespoons butter
6 Tablespoons flour
2 cups milk
1 cup half & half
2 teaspoons salt
1 teaspoon dry, ground mustard
pepper to taste
3 cups grated cheddar cheese , divided


INSTRUCTIONS
Preheat oven to 350 degrees. Grease a 9x13 pan.
Heat a pan of water to boil.
Peel, wash, and thinly slice potatoes, then add them to the boiling water and cook them until they are al dente - 5 to 7 minutes.
Drain the potatoes and transfer them to the baking pan.
If you are adding the onion, dice it and saute it in a small amount of butter.
In a sauce pan, combine the butter, flour, milk, half & half, and seasonings.
Cook sauce on low until smooth and boiling, stirring with a whisk.
Reduce heat and stir in the onion and 1 1/2 cups of the cheddar cheese.
Place a half of the sliced potatoes in the baking dish. Pour half of cheese sauce over potatoes. Repeat with second layer of potatoes and cheese sauce.
Sprinkle the remaining cheese on top.


Bake uncovered for 25-40 minutes at 375 degrees.