Cooking Steps

• Heat 3 Tbsp. extra-virgin olive oil in a large high-sided ovenproof skillet over medium-high heat. Add 4 large yellow onions, a pinch of kosher salt, and a pinch of freshly ground black pepper. Cook until onions are deep mahogany color and jammy, about 30–35 minutes.
• Add 6 finely chopped garlic cloves and 4 tsp. chopped thyme, cooking until fragrant, about 1 minute. Then, add ¾ cup dry white wine and reduce by half, about 3 minutes.
• Add 5½ cups water and bring to a simmer. Stir in 1 lb. lumache (or other medium shell pasta) and cook until almost all liquid is absorbed and pasta is al dente, about 10–14 minutes.
• Remove pot from heat and add 1 oz. Parmesan, 5 Tbsp. unsalted butter, 1 Tbsp. Dijon mustard, 2 tsp. sugar, 2 tsp. Worcestershire sauce, remaining salt, and remaining black pepper. Stir until cheese is melted.
• Place a rack in upper third of oven and heat broiler. Scatter 8 oz. Gruyère over pasta and broil until melted and golden brown, about 2–5 minutes.
• Top with thinly sliced chives if using just before serving.