1. Preheat your oven to 375°F (190°C). Generously grease a 9×13 inch baking dish with butter.

2. In a large saucepan, melt the 4 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 minute to create a roux.

3. Gradually whisk in the warm milk and heavy cream until the mixture is smooth. Bring to a simmer and cook for 2-3 minutes, whisking constantly, until the sauce thickens.

4. Remove the saucepan from the heat. Stir in the Dijon mustard, nutmeg, salt, and pepper. Gradually stir in 1 cup of the shredded cheddar and all of the Parmesan cheese until melted and the sauce is creamy.

5. Arrange half of the sliced potatoes in an even layer in the bottom of the prepared baking dish. Top with half of the sliced onions.

6. Pour half of the cheese sauce evenly over the potatoes and onions.

7. Repeat the layers with the remaining potatoes and onions, and top with the remaining sauce.

8. Sprinkle the remaining 1/2 cup of cheddar cheese over the top.

9. Cover the dish tightly with aluminum foil and bake for 45 minutes.

10. Remove the foil and bake for another 25-30 minutes, or until the potatoes are tender when pierced with a fork and the top is bubbly and golden brown.

11. Let the dish rest for at least 15 minutes before serving. Garnish with fresh thyme or parsley.