Step 1

Freeze 1 stick cold unsalted butter until firm, about 15 minutes. Meanwhile, whisk 1 1/2 cups all-purpose flour, 1 teaspoon black pepper, and 1/2 teaspoon kosher salt together in a large bowl.

Step 2

Place a box grater over a small piece of parchment paper, cutting board, or large plate. Grate the butter on the large holes of a box grater. When you get down to a small nub of butter, chop that nub into 5 to 6 small pieces. Reserve the grater (no need to clean).

Step 3

Transfer the butter to the dry ingredients and reserve the parchment paper. Use your fingers to toss the butter in the flour, breaking up any clumps, until evenly coated.

Step 4

Drizzle 1/4 cup plus 3 tablespoon ice water over the butter-flour mixture. Use a fork or your hands to combine and lightly mix until a rough dough ball forms; do not overwork the dough.

Step 5

Turn the dough out onto a large piece of plastic wrap. Use the palm of your hand to quickly gather and press the mound into a thick disk. Wrap in the plastic wrap and refrigerate while you prepare the filling, at least 1 hour.

Step 6

Halve and thinly slice 3 pounds yellow onions (about 11 cups). Strip the leaves from 10 fresh thyme sprigs.

Step 7

Melt 3 tablespoons unsalted butter in a 12-inch or larger, preferably high-sided, 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, 45 to 60 minutes. Meanwhile, grate 6 ounces Gruyère cheese on the large holes of the box grater onto the reserved parchment paper (about 1 1/2 cups).

Step 8

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 from the heat and stir in 2 teaspoons balsamic or sherry vinegar.

Step 9

Arrange a rack in the middle of the oven and heat the oven to 400°F. Prepare a sheet of parchment paper about the size of a baking sheet. Lightly dust the parchment with all-purpose flour.

Step 10

Unwrap the dough and place it on the prepared parchment paper. Sprinkle the dough with more flour and, using a rolling pin, roll it out into a round about 12 inches wide. Carefully transfer the parchment paper, with the rolled-out dough on it, onto a baking sheet.

Step 11

Spread 2 teaspoons Dijon mustard evenly onto the dough with the back of a spoon or a butter knife, leaving a 1 1/2- to 2-inch border. Sprinkle half of the cheese onto the mustard. Spoon the caramelized onions onto the cheese.

Step 12

Gently fold the edges of the dough over the filling, pleating it about every 2 inches. Sprinkle the remaining cheese on top. (Don’t worry if some of the cheese gets on the crust — it will just result in some nice, golden-brown crispy bits like French onion soup!)

Step 13

Bake until the crust is firm and golden-brown, and the cheese is melted and browned in spots, about 35 minutes. Let the galette cool for at least 5 to 10 minutes before serving. Garnish with the remaining thyme leaves and serve warm or at room temperature, cut into wedges.