Preheat your oven to 375°F (190°C) and grease a baking dish with unsalted butter.

Wash and thinly slice the potatoes, placing them in cold water to prevent browning.

Melt 4 tablespoons of butter in a saucepan, whisk in flour, and cook for 2 minutes to form a roux.

Add milk and heavy cream slowly, whisk continuously until the sauce thickens (8-10 minutes).

Stir in onion powder, garlic powder, salt, and pepper, then let simmer for 2-3 minutes.

Remove from heat and stir in mozzarella and cheddar until melted and smooth.

Pour half the cheese sauce over the potatoes in the baking dish, cover with foil, and bake for 30 minutes.

Uncover and add the remaining sauce over the potatoes; bake uncovered for an additional hour until golden brown.

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