• Preheat the oven to 350°. Grease a 10x15 pan and set aside.
• Heat a large pot 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.
• If you are adding the onion, dice it and saute it in a small amount of butter.

Sauce:

• While the potatoes are cooking, prepare the sauce.
• In a saucepan, combine the cold butter, flour, milk, and seasonings.
• Bring it to a boil over medium heat, stirring constantly so it won't scorch.
• When the sauce has come to a boil and is smooth, add the sauteed onion and 2 cups of shredded cheese.
• Remove from heat and set aside. The sauce will be thick, but it will turn into the perfect consistency and ratio when the casserole is baked.

When the potatoes are done:

• Drain them and transfer 1/2 of them to the prepared baking dish, spreading them out evenly. Pour half of the sauce over the potatoes and repeat the process.
• Sprinkle the remaining 2 cups of cheese over the potatoes, making sure the entire top layer is covered with either cheese or sauce so the potatoes don't dry out.
• Bake uncovered until heated through - 20-30 minutes.