French Onion Mac and Cheese

In a large skillet over medium heat, melt 2 1/2 tablespoons butter. Add the onions with a good pinch of salt. Sauté, stirring occasionally and reducing the heat if necessary, until the onions have completely softened and caramelized, 30 minutes.
Once the onions are cooked, pour in the wine and add most of the thyme leaves, reserving a handful for garnish. Simmer until the wine has reduced by half, 2 to 4 minutes. The onions can be prepared in advance and kept in the fridge.
In a large saucepan of boiling salted water, cook macaroni for 2 minutes less than the package instructions instruct, then drain in a colander.

In a large ovenproof skillet over medium heat, add remaining 4 1/2 tablespoons butter. Tip in the flour and whisk to combine. Whisking constantly, slowly pour the cold milk into the flour mixture, a little at a time, to get a smooth sauce. Simmer the sauce until thickened, about 5 minutes.
Stir in Cheddar cheese and half of the Gruyère cheese. Once the cheese has melted, take sauce off the heat, add Dijon mustard, and season with salt and black pepper.
Preheat broiler to high heat.
Tip the cooked macaroni and caramelized onions into the cheese sauce, reserving a tablespoon of the onions. Mix well, then top with remaining onions, Gruyère cheese, and the breadcrumbs. Scatter over a handful more thyme leaves.
Broil until golden brown, 2 to 4 minutes. Serve immediately.