Prep Oven & Dish: Preheat your oven to 375°F (190°C). Lightly grease a baking dish with a touch of butter.
Slice Potatoes: Wash your potatoes thoroughly. You can peel them if you like, but leaving the skin on adds fiber and nutrients! Slice them very thinly (a mandoline makes this super easy and quick). Arrange these slices in an even layer in your prepared baking dish.
Make the Light Cream Sauce: In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for just a minute or two, until it forms a smooth paste. Slowly pour in the milk and cream, whisking continuously to keep it lump-free. Stir in the onion powder, garlic powder, salt, and pepper. Bring this mixture to a gentle boil, then reduce the heat and let it simmer for about 10 minutes, stirring occasionally, until it thickens nicely.
Melt the Cheese: Remove the saucepan from the heat and stir in both the mozzarella and cheddar cheeses until they are completely melted and the sauce is smooth and luxurious.
Layer & Bake (Initial): Pour half of this wonderful cheesy sauce evenly over your prepped potato layer in the baking dish. Cover the dish tightly with aluminum foil. Bake in the preheated oven for 30 minutes.
Second Bake & Finish: Carefully remove the foil. Pour the remaining cheesy sauce over the partially baked potatoes. Return the dish to the oven, uncovered, and bake for another hour, or until the potatoes are perfectly tender and the top is beautifully golden and bubbly.
Serve with Freshness: Let the dish cool for a few minutes before serving. Sprinkle generously with fresh chopped parsley or dill for a vibrant pop of color and flavor. Enjoy this comforting and flavorful side dish!