<p>4 medium russet potatoes, thinly sliced</p><p>1 medium onion, sliced into rings</p><p>salt and ground black pepper to taste</p><p>3 tablespoons butter</p><p>3 tablespoons all-purpose flour</p><p>½ teaspoon salt</p><p>2 cups milk</p><p>1 ½ cups shredded Cheddar cheese</p>
<p>Gather all ingredients. Preheat the oven to 400 degrees F (200 degrees C). Butter a 2-quart casserole dish.</p><p>Layer 1/2 of the potatoes in the bottom of the prepared casserole dish; season with salt and pepper.<br></p><p>Layer onion slices over top, then top with with remaining potatoes. Season again with salt and pepper.<br></p><p>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.<br></p><p>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.<br></p><p>Cook, whisking constantly, until the mixture has thickened, 3 to 5 minutes.<br></p><p>Stir in cheese all at once; continue stirring until melted, 30 to 60 seconds.<br></p><p>Pour cheese sauce over the potatoes, and cover the dish <br></p><p>Bake in the preheated oven until potatoes are tender and sauce is bubbly, about 1 ½ hours.</p><p><br></p>