Summarize this article in one sentence.
Prepare a 9-inch (23-cm) baking dish by spraying it with nonstick cooking spray.  You can use a square or oval dish. If you do not have cooking spray, use butter or shortening. Spread the baking spray, butter, or shortening around the dish using a clean paper towel to ensure that the dish is well coated. Add the butter to a medium skillet with high sides and heat over medium heat. Wait until the butter is melted before proceeding to the next step. Add the onion to the melted butter and cook, stirring frequently, until it softens.  The onions may also begin to turn translucent, but they should not be allowed to brown or caramelize. Use a spatula or mixing spoon to stir the onions as they cook. Note that this should only take 2 minutes or so. Sprinkle the flour over the onions and whisk the ingredients together. Cook for 2 additional minutes.  Cooking the flour at this stage prevents the taste of the raw flour from affecting the taste of the sauce. Evenly sprinkle the flour over the onions to minimize the risk of clumps forming. Whisk the milk into the skillet, combining it thoroughly with the onions and flour. Continue cooking until the sauce thickens and bubbles. It should only take a few minutes for the sauce to thicken. Stir frequently with your whisk to check the texture and prevent the sauce from burning to the bottom of the pan. Remove the skillet from the heat and add the cheese. Use a mixing spoon to stir it into the sauce until it melts and distributes itself evenly. Season with salt and pepper, if desired, mixing the seasonings into the sauce thoroughly. Place half of your sliced potatoes in the prepared baking dish and sprinkle with additional salt and pepper, if desired. The potatoes will not be in a single layer, but the layers in the bottom of the dish should be even in order to ensure even cooking. Spoon half of your prepared sauce over the potatoes in the baking dish.  Spoon the sauce over the potatoes as evenly as possible. Give the baking dish a slight shake from side to side to help settle the sauce in between the potato layers. Layer the remaining sliced potatoes over your sauce and pour the remaining sauce over them.  Both the potatoes and sauce should be distributed evenly. Give the dish a slight shake from side to side to settle the contents. Evenly distribute the remaining 1/2 cup (125 ml) shredded cheese over the top of the dish.  You can use shredded or grated cheese. Grated cheese will melt into the sauce more, while shredded cheese will create a layer of cheese over the dish. Cheddar cheese is a popular choice, but most other varieties will also work. For instance, you could use Parmesan cheese or a commercial blend of Mexican cheeses and spices. Cover the dish with aluminum foil and bake in your preheated oven until the potatoes are tender. If you want the cheese to brown on top, remove the foil during the last 5 to 10 minutes. Let the potatoes cool for a couple of minutes after you pull them out of the oven, but serve them while still hot and bubbling.

Summary:
Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius). Heat the butter in a skillet. Cook onion until soft. Whisk in flour. Whisk in milk. Add 3/4 cup (180 ml) cheese. Arrange half of the potatoes in your baking dish. Cover with half of the sauce. Repeat with the remaining potatoes and sauce. Sprinkle with remaining cheese. Bake for 30 minutes. Serve hot.