Buttermilk Parmesan Potatoes

Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x13-inch baking dish.

Melt 1 tablespoon butter in a saucepan over medium heat. Add onion and garlic; cook and stir until onion is translucent, about 5 minutes. Stir in remaining 3 tablespoons butter until melted. Sprinkle flour, salt, and pepper over onion mixture; cook and stir until mixture bubbles and thickens, about 2 minutes. Gradually whisk in milk and buttermilk, and return mixture to a boil. Stir in 3/4 cup Parmesan cheese; cook until melted and smooth, about 2 minutes.

Arrange potatoes over the bottom of the prepared baking dish. Pour sauce over potatoes, and stir until evenly coated.

Bake in the preheated oven for 1 hour, stirring halfway through. Stir once more, top with remaining 1/4 cup Parmesan, and bake until cheese is melted and golden brown, about 20 more minutes.