Preheat the oven to 375°F.
Wash the potatoes and slice them thinly and evenly.
Arrange the potato slices in a baking dish.
In a saucepan, whisk together milk, cream, melted butter, flour, garlic powder, onion powder, salt, and black pepper.
Cook over medium heat, whisking often, until the sauce thickens slightly.
Stir in some of the mozzarella and cheddar until melted.
Pour the sauce over the potatoes, making sure it goes between the slices.
Cover the baking dish with foil and bake for 45 minutes.
Remove the foil, sprinkle the remaining cheese on top, and bake for 20 to 25 minutes more.
Bake until the potatoes are tender and the top is golden and bubbly.
Let the dish rest for 10 minutes before serving.
Garnish with parsley or dill and serve warm.