For the Crust
1 1/2 cups all-purpose flour
1/4 teaspoon salt
1/2 teaspoon ground white pepper
8 tablespoons (1 stick) cold unsalted butter, cut into small pieces
4 tablespoons ice water
1 egg beaten with 1 tablespoon water
Combine dry ingredients in the bowl of a standing mixer. Using a small whisk, mix breifly until the mixture is aerated. Cut the butter into the dry mixture using a pastry cutter or your fingers until the butter and flour are thoroughly combined. The resulting mixture should be crumbly, pea-sized pieces.
Transfer your bowl to your standing mixer and drizzle in the ice water, mixing until the dough comes just together. Shape the dough into a ball, cover with plastic wrap and refrigerate for at least an hour before using.
For the Filling
1 tablespoon unsalted butter
1 tablespoon extra-virgin olive oil
2 yellow onions, sliced into thin rings
1/4 cup crème fraîche
1 teaspon Dijon mustard
1 1/2 cups shredded Gouda cheese
1 1/2 tablespoons all-purpose flour
2 teaspoons fresh thyme leaves, finely chopped, plus more for garnish (optional)
3 or 4 baking apples, cored and thinly sliced
salt and freshly ground pepper to taste
fresh thyme leaves
Preheat oven to 375 degrees. Line a baking sheet with parchment paper.Flour your working surface and use a rolling pin to flatten the dough into a round disk (about 12″). In a large fry pan over medium heat, melt the butter with the olive oil. Add the onions and cook until lightly browned and starting to caramelize, about 8 minutes per side. Remove from the heat. In a small bowl, stir together the crème fraîche and mustard.
Transfer your dough to your baking sheet. Spread the crème fraîche and mustard mixture in the center of the dough leaving a 2-3 in border all around. Sprinkle 1 cup of the cheese and the thyme on top. Then arrange the apple slices and onions on top. Sprinkle with the remaining 1/2 cup cheese, and season with salt and pepper. Fold the galette edges over the filling, leaving the center open and filling exposed, overlapping as necessary. Press gently to seal the edges.Brush the galette crust with the beaten egg. Sprinkle salt and pepper over the top of the galette. Bake on the top oven rack until the pastry is golden and the cheese is melted, about 25 minutes. Let cool for 10 minutes before serving. Garnish with fresh thyme.