Preheat the oven to 375°F (190°C)., Lightly grease a baking dish with butter or cooking spray., In a saucepan, melt the butter over medium heat., Add the chopped onion and minced garlic, cooking until soft and translucent., Stir in the flour and cook for about 1-2 minutes to form a roux., Gradually whisk in the milk and heavy cream, stirring continuously until the sauce thickens (about 5-7 minutes)., Add the shredded cheddar cheese and stir until melted and smooth. Season with salt and black pepper., Arrange half of the potato slices in the bottom of the prepared baking dish., Pour half of the cheese sauce over the first layer of potatoes., Repeat with the remaining potatoes and top with the remaining sauce., Cover the dish with aluminum foil and bake for 40 minutes., Remove the foil and bake for an additional 20-30 minutes, or until the top is golden brown and the potatoes are fork-tender., Allow the dish to sit for about 10-15 minutes before serving.