Triple-Cheese French Onion Mac & Cheese

Instructions

1. 1In a large skillet over medium-low heat, melt 2 tablespoons of the butter. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, until the onions are deeply caramelized and jammy, which should take about 25 to 30 minutes. Stir in the fresh thyme during the last minute of cooking, then remove from heat.
2. 2Preheat your oven to 375°F and lightly grease a 9x13-inch baking dish or a 3-quart oven-safe casserole dish.
3. 3Bring a large pot of heavily salted water to a boil. Cook the pasta according to package directions but stop 2 minutes before it reaches al dente. Drain the pasta and set it aside.
4. 4In the same large pot used for the pasta, melt the remaining 2 tablespoons of butter over medium heat. Whisk in the flour and cook for about 1 minute to remove the raw flour taste, ensuring it doesn't brown too much.
5. 5Slowly whisk in the milk and heavy cream, a little at a time, to avoid lumps. Continue to simmer, whisking frequently, until the sauce thickens enough to coat the back of a spoon, approximately 5 to 7 minutes.
6. 6Remove the pot from the heat. Whisk in 1 cup of the Gruyère, all of the white cheddar, and all of the Parmesan until the cheese is completely melted and the sauce is silky. Stir in the Dijon mustard, nutmeg, salt, and pepper.
7. 7Gently fold the cooked pasta and the caramelized onions into the cheese sauce until every noodle is well-coated.
8. 8Pour the mixture into the prepared baking dish, spreading it out evenly.
9. 9In a small mixing bowl, toss the sourdough breadcrumbs with the olive oil and the remaining 1 cup of Gruyère cheese. Sprinkle this mixture evenly over the top of the mac and cheese.
10. 10Bake for 20 to 25 minutes until the sauce is bubbling at the edges and the breadcrumb topping is a deep golden brown. Let the dish rest for 5 minutes before serving to allow the sauce to set.

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