Scalloped Potatoes

Instructions

1. Prepare the oven and baking dish
   Preheat the oven to 350°F (175°C). Butter a 9x13 inch baking dish or similar 3-quart casserole dish. Set aside.
2. Make the béchamel sauce
   In a large saucepan, melt the butter over medium heat. Add the minced garlic and cook for 30 seconds until fragrant. Whisk in the flour and cook for 1-2 minutes until it forms a pale roux. Gradually whisk in the warm milk and cream, stirring constantly to prevent lumps. Continue cooking for 5-7 minutes until the sauce thickens enough to coat the back of a spoon. Remove from heat and stir in 1/2 cup of Gruyère, nutmeg, salt, and pepper.
3. Layer the potatoes and onions
   Arrange one-third of the sliced potatoes in an overlapping pattern in the prepared baking dish. Scatter half of the sliced onions on top. Pour one-third of the béchamel sauce over the potatoes and onions. Repeat with another layer of potatoes, the remaining onions, and another third of the sauce. Finish with the remaining potatoes and sauce.
4. Add cheese topping
   Sprinkle the remaining 1 cup of Gruyère cheese and the Parmesan cheese evenly over the top. Scatter the fresh thyme leaves over the cheese.
5. Bake the potatoes
   Cover the dish with aluminum foil and bake for 45 minutes. Remove the foil and continue baking for another 30 minutes, or until the potatoes are tender when pierced with a knife and the top is golden brown and bubbling.
6. Rest and serve
   Remove from the oven and let stand for 10-15 minutes before serving. This allows the sauce to set slightly and makes it easier to serve neat portions.