Cooking Steps

• Heat 3 Tbsp. extra-virgin olive oil in a large high-sided ovenproof skillet over medium-high heat. Add the onions, a pinch of kosher salt, and a pinch of freshly ground black pepper and cook, stirring occasionally until onions are a deep mahogany color and jammy, about 30–35 minutes.
• Add the garlic and chopped thyme and cook until fragrant, about 1 minute. Add the white wine and cook until reduced by about half, about 3 minutes. Add 5½ cups water and bring to a simmer. Add the pasta and cook until almost all the liquid is absorbed and pasta is al dente, about 10–14 minutes.
• Remove from heat and add Parmesan, butter, Dijon mustard, sugar, Worcestershire sauce, remaining salt, and remaining black pepper, stirring until cheese is melted.
• Place a rack in the upper third of the oven and broil the Gruyère over the pasta until melted and golden brown in spots, about 2–5 minutes depending on your broiler.
• Top with thinly sliced chives just before serving if using.