Preparation Steps

• Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish with butter or non-stick spray.
• In a large saucepan over medium heat, melt butter. Add onions and cook until softened, about 5 minutes.
• Whisk in flour and cook for 1-2 minutes to form a roux, stirring constantly.
• Gradually whisk in milk and cream until smooth. Cook, stirring frequently, until the sauce thickens and coats the back of a spoon, about 5-7 minutes.
• Remove from heat and stir in salt, pepper, garlic powder, nutmeg (if using), and half of the cheddar cheese until melted.
• Layer half of the sliced potatoes in the prepared baking dish. Pour half of the cheese sauce over the potatoes. Repeat with remaining potatoes and sauce.
• Sprinkle the remaining cheddar cheese and Parmesan cheese evenly over the top.
• Cover with aluminum foil and bake for 45 minutes. Remove foil and bake for an additional 15 minutes, or until the top is golden and the potatoes are tender when pierced with a fork.
• Let rest for 10 minutes before serving to allow the sauce to set slightly.