Begin by peeling and thinly slicing your potatoes into 1/8-inch rounds. Thinly slice the onion. Preheat your oven to 400°F (200°C) and grease a 9x13 inch baking dish.

Arrange a layer of potato slices in the greased baking dish, overlapping them slightly. Sprinkle with salt and black pepper. Layer with sliced onions, then more potatoes and seasoning.

In a medium saucepan over medium heat, melt the butter. Whisk in the flour and salt to form a roux; cook for about 1 minute. Gradually pour in the milk, whisking continuously for 5-7 minutes until thickened.

Remove from heat and stir in the cheddar cheese until melted and creamy.

Pour the cheese sauce evenly over the layered potatoes and onions. Cover tightly with aluminum foil.

Bake in the preheated oven for 85-90 minutes. The edges should bubble, indicating the potatoes are tender.

Remove the foil, switch the oven to broil, and broil for 2-4 minutes until golden brown.

Allow to rest for about 5 minutes before serving to enhance flavors.