Preheat your oven to 375°F (190°C).

Grease a 9×13-inch baking dish with butter or non-stick spray.

In a medium saucepan over medium heat, melt 3 tablespoons of butter until frothy. Gradually whisk in 3 tablespoons of all-purpose flour, cooking for 1-2 minutes until golden and bubbly.

Slowly whisk in 1 ½ cups of milk into the roux, stirring continuously as you bring the mixture to a gentle boil. Reduce heat and let simmer for 5-7 minutes until thickened.

Remove from heat and stir in 1 ½ cups of grated sharp cheddar cheese until melted. Season with salt and cayenne pepper if desired.

Layer half of your sliced potatoes in the baking dish, pour half of the cheese sauce over them.

Repeat the layering with the remaining potatoes and cheese sauce.

Sprinkle ½ cup of grated cheddar or Gruyère on top.

Cover with foil and bake for 45 minutes. Then remove foil and bake for another 15-20 minutes until golden.

Allow to cool for 10-15 minutes before serving.