STEP 1 Preheat the oven to 400 degrees F (200 degrees C). Butter a 1-quart casserole dish. STEP 2 Layer 1/2 of the potatoes in the bottom of the prepared casserole dish; season with salt and pepper. Layer onion slices over top, then top with with remaining potatoes. Season again with salt and pepper. STEP 3 Melt butter in a medium saucepan over medium heat. Gradually whisk in flour and salt and cook, whisking constantly until raw flour flavor has cooked off, about 1 minute. Gradually add milk, about 1/4 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. STEP 4 Cook, whisking constantly, until the mixture has thickened, 3 to 5 minutes. Stir in cheese all at once; continue stirring until melted, 30 to 60 seconds. Pour cheese sauce over the potatoes, and cover the dish with aluminum foil. STEP 5 Bake in the preheated oven until potatoes are tender and sauce is bubbly, about 1 1/2 hours.