Heat 3 Tbsp. of extra-virgin olive oil in a large (3½-qt. capacity or more) high-sided ovenproof skillet over medium-high heat.

Add the thinly sliced onions, a pinch of kosher salt, and a pinch of black pepper. Cook the onions, stirring occasionally and adding 1 Tbsp. of water to prevent burning, until they become a deep mahogany color, about 30–35 minutes.

Once the onions are caramelized, add the finely chopped garlic and chopped thyme. Cook until fragrant, about 1 minute.

Pour in ¾ cup of dry white wine, cooking until it reduces by half, about 3 minutes.

Then, add 5½ cups of water and bring to a simmer.

Add 1 lb. of lumache (snail shells) or another medium shell pasta, stirring often to prevent sticking, and cook until the pasta has absorbed most of the liquid and is al dente, about 10–14 minutes.

Remove the pot from heat. Stir in 1 oz. of finely grated Parmesan, 5 Tbsp. of unsalted butter, 1 Tbsp. of Dijon mustard, 2 tsp. of sugar, 2 tsp. of Worcestershire sauce, the remaining salt, and pepper until the cheese melts.

Preheat the broiler with a rack in the upper third of the oven.

Sprinkle 8 oz. of coarsely grated Gruyère over the pasta and broil until melted and golden brown, about 2–5 minutes, depending on your broiler.

Serve topped with thinly sliced chives, if desired.