Preheat your oven to 375°F (190°C) to ensure even baking. This step helps create a golden-brown topping.

Lightly grease a 9x13-inch baking dish with butter or non-stick spray to prevent sticking and make cleanup easier.

Melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1–2 minutes until it turns a light golden color. Gradually pour in the milk, whisking constantly to avoid lumps. Bring to a simmer and cook for 5–7 minutes until thickened. Remove from heat and stir in grated cheddar cheese. Add salt to taste and cayenne pepper if desired.

Arrange half of the thinly sliced potatoes in the greased baking dish, overlapping slightly. Pour half of the cheese sauce over the potatoes, spreading evenly. Add the remaining potatoes in an even layer, then pour the remaining cheese sauce on top.

Sprinkle grated cheddar or Gruyère cheese evenly over the top to create a crispy, golden-brown crust during baking.

Cover the dish with aluminum foil and bake for 45 minutes to allow the potatoes to cook through. Remove the foil and bake for an additional 15–20 minutes until the top is golden brown and bubbling.

Let the scalloped potatoes rest for 10–15 minutes to allow the sauce to set. Optionally, garnish with sliced green onions for a fresh touch.