Step 1

Halve and thinly slice 2 pounds yellow onions (about 7 cups). Pick the leaves from 1 small bunch fresh thyme sprigs until you have 2 tablespoons.

Step 2

Heat 1 tablespoon of the olive oil in a 12-inch cast iron or other broiler-safe, heavy-bottomed skillet over medium heat until shimmering. Break up 1 (16- to 18-ounce) package potato gnocchi into individual gnocchi and add to the skillet in a single layer. Cover (​​a sheet pan or the lid of a Dutch oven works well) and cook undisturbed until the bottoms are golden-brown, 3 to 5 minutes. Transfer to a rimmed baking sheet and spread out in a single layer. Repeat with the remaining 1 tablespoon olive oil and 1 package gnocchi and transfer it to the baking sheet.

Step 3

Melt 2 tablespoons unsalted butter in the now-empty skillet over medium heat. Add the onions, half of the thyme leaves, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Cook, stirring frequently with a wooden spoon and reducing the heat if the onions start to burn, until the onions have reduced by over half, are very soft, and deep golden -brown, 28 to 30 minutes. Meanwhile, grate 6 ounces Gruyère cheese on the large holes of the box grater (about 1 1/2 cups). Arrange a rack 6 to 8 inches below the broiling element and set the oven to broil.

Step 4

Pour 1/4 cup dry white wine over the caramelized onions. Scrape up any browned bits on the bottom of the pan and continue to stir until the wine has evaporated, about 1 minute. Remove the pan from the heat and stir in 1 teaspoon balsamic or sherry vinegar.

Step 5

Return the gnocchi to the skillet and stir until combined. Taste and season with more kosher salt as needed. Spread the mixture out into an even layer. Sprinkle with the cheese.

Step 6

Transfer the pan to the oven and broil until the cheese is melted, bubbly, and browned in spots, 4 to 6 minutes. Garnish with the reserved thyme leaves and several grinds of black pepper.