Step 1

To make the caramelized onion, cut the onion in half lengthwise, then lengthwise into 1-inch (2.5-cm) wedges, keeping the root end intact.

Step 2

In a large fry pan over medium heat, melt the butter. Add the onion and cook without stirring until the wedges start to brown and stick to the pan, about 3 minutes. Sprinkle with a pinch of sugar and season with salt and pepper. Add the sherry and cook until almost evaporated, about 1 minute, then stir the onion, scraping up the caramelized brown bits. Cook without stirring until the onion is nicely browned, about 3 minutes, then add about 2 Tbs. broth, stirring and scraping up the caramelized brown bits. Cook without stirring until the onion starts to brown again, about 3 minutes, then add about 2 Tbs. broth, stirring and scraping up the caramelized brown bits. Repeat the process until you’ve used up all of the broth. Remove from the heat and stir in the thyme sprigs. Let cool.

Step 3

Preheat an oven 375°F (190°C). Line a baking sheet with parchment paper.

Step 4

On a lightly floured surface, roll out the puff pastry as needed into a 14-inch (35-cm) round. Transfer to the prepared baking sheet. Brush with the mustard, leaving a 2-inch border uncovered, then sprinkle the cheese over the mustard. Arrange the onion wedges on top. Fold the dough up and over the filling, forming loose pleats all around the edge and leaving the center open. Lightly brush the pleated dough with the egg mixture and sprinkle with salt. Refrigerate the galette for 15 to 20 minutes.

Step 5

Bake until the crust is deep golden brown and the onion is deeply caramelized, 40 to 45 minutes. Transfer the baking sheet to a wire rack and let cool for 10 minutes. Garnish the galette with chives, thyme leaves and a few grindings of pepper. Cut into slices and serve. Serves 8.

Step 6

Williams Sonoma Test Kitchen