Caramelized Onion and Gruyère Galette

Make the rye dough

1. In a small bowl, mix together egg yolks, vinegar, and 8 tablespoons of ice water.
2. In a large bowl, combine the all-purpose and rye flours and salt. Add the butter, and toss to coat. Using your hands, pinch the pieces of butter between your thumbs and pointer fingers to flatten them into thin shards of butter, toss to fully coat in flour. Continue until all the butter has been flattened, working quickly to prevent butter from getting too warm.
3. Pour the egg mixture over the flour mixture, toss with your hands to incorporate the ingredients.
4. Drizzle over the remaining 1 to 2 tablespoons of ice water as needed, a little at a time, just until the dough comes together and is no longer dry and crumbly. You may not need all of the remaining 2 tablespoons, depending on the humidity of the day, or you may need a bit more. You want to add just enough water for the dough to feel firmly held together and smooth rather than dry and crumbly, but not so much that it’s soft, wet, and sticky to the touch.
5. Pat dough out into a disc about 1-inch (2.5 cm) thick. Using a bench scraper or knife, slice disc into quarters and stack pieces on top of one another. Gently pat out the dough with your hands into a disc about 1-inch (2.5 cm) thick. Repeat the cutting and stacking once more. This process is going to give you all those flaky layers.
6. Divide dough into 2 discs, wrap tightly in plastic wrap and chill for at least 2 hours or, ideally, overnight. Allow dough to sit at room temperature for a few minutes to soften slightly before rolling it out. You only need one disc for this recipe, so the remaining disc can be frozen for future pie or galette cravings.

Make the caramelized onion and gruyère galette

1. Heat a large cast-iron skillet over medium-low heat. Add olive oil and butter to the skillet. Once the butter has melted, add the onion rounds in a single layer and tuck the whole sprigs of rosemary and thyme into the skillet. Season the onion with salt and pepper and let cook, undisturbed, until the onion is caramelized and golden brown on the bottom, 10 to 15 minutes.
2. Using a small offset spatula, carefully flip, being careful to keep the onions in one piece, season with salt and pepper again, and cook until there’s a caramelized layer on the other side, 10 to 15 minutes. Should you have a few layers of onion slip around a bit, no matter. Just tuck them in their round as best you can.
3. Add wine and beef broth, and cook until the liquid has reduced and is thick and syrupy, 8 to 10 minutes. Remove skillet from heat and let onions cool completely.
4. In a small bowl, toss the Gruyère and Parmigiano cheeses together with the chopped thyme and rosemary.
5. Line a rimmed baking sheet with parchment paper. On a well-floured surface, roll the dough out into a large circle 1/4- to 1/8-inch (6 to 3 mm) thick and 10- to 12-inches (25 to 30 cm) in diameter. Transfer dough to the prepared baking sheet.
6. Sprinkle cheese mixture on dough, leaving about a 2-inch (5-cm) border. Gently arrange the onion rounds over the cheese mixture in a single layer. Spoon the syrupy pan juices over the onions.
7. Fold the sides of the dough up and over the edge of the filling, overlapping it onto itself to create a pleated pattern. Freeze the galette on the baking sheet for 30 minutes.
8. Meanwhile, preheat oven to 400°F (204°C).
9. Brush the sides of the dough with beaten egg, and sprinkle with flaky salt. Bake until the crust is golden brown and the filling is bubbling, 30 to 35 minutes. Cool the galette for a few minutes before serving.