Preparation

• Gather all ingredients. Preheat the oven to 400 degrees F (200 degrees C). Grease a 2-quart casserole dish with butter.
• Layer ½ of the potatoes in the bottom of the prepared casserole dish; season with salt and pepper.
• Layer onion slices over top, then top with remaining potatoes. Season again with salt and pepper.
• Melt butter in a medium saucepan over medium heat. Whisk in flour and salt; cook, whisking constantly, until raw flour flavor has cooked off, about 1 minute.
• Gradually add milk, about ¼ cup at a time, whisking well after each addition to incorporate; the gradual addition and whisking of milk will help avoid lumps in your sauce.
• Cook, whisking constantly, until the mixture has thickened, 3 to 5 minutes.
• Stir in cheese all at once; cook, stirring constantly, until melted, 30 to 60 seconds.
• Pour cheese sauce over the potatoes, and cover the dish with aluminum foil.
• Bake in the preheated oven until potatoes are tender and sauce is bubbly, about 1 ½ hours.