French Onion Soup Mac and Cheese

Preheat oven to 375°F (190°C). Grease a 9x13 inch baking dish.

Cook macaroni according to package directions until al dente. Drain and set aside.

While the pasta cooks, melt 2 tablespoons of butter in a large skillet over medium-low heat. Add sliced onions and cook, stirring occasionally, until deeply caramelized, about 25-30 minutes. Deglaze the pan with sherry (if using) during the last 5 minutes of cooking, scraping up any browned bits. Set aside.

In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Whisk in flour and cook for 1-2 minutes, stirring constantly, until a smooth paste forms (a roux).

Gradually whisk in milk, ensuring no lumps form. Bring to a simmer, stirring constantly, and cook for 5-7 minutes, or until the sauce has thickened slightly.

Remove from heat and stir in heavy cream, 6 ounces of shredded Gruyere cheese, cheddar cheese, Dijon mustard, salt, and pepper. Stir until cheeses are melted and the sauce is smooth.

Add cooked macaroni and caramelized onions to the cheese sauce. Stir to combine well.

Pour the mixture into the prepared baking dish. Top with the remaining 2 ounces of Gruyere cheese and sprinkle bread crumbs on top.

Bake for 20-25 minutes, or until golden brown and bubbly.

Let cool for a few minutes before serving.