Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish with butter or cooking spray.

Peel the potatoes if desired, and thinly slice them to about 1/8-inch thick. Soak in cold water to prevent browning.

In a medium saucepan, melt butter over medium heat. Add flour and whisk until smooth. Gradually pour in milk and cream, whisking to avoid lumps. Season and bring to a boil, then simmer until thickened.

Stir in mozzarella and cheddar until melted and smooth.

Layer half of the potato slices in the baking dish, pour half of the cheese sauce over them. Repeat the layers.

Cover with foil and bake for 30 minutes, then remove foil and bake for an additional hour until golden and bubbly.

Let rest for 10 minutes, then garnish with chopped parsley or dill before serving.