Ingredients

• 1 cup whole-wheat flour
• 3/4 cup all-purposed flour, plus more for dusting
• 1 1/2 teaspoons kosher salt, divided
• 1/3 cup plus 3 tablespoons olive oil, divided
• 1/3 cup water
• 2 teaspoons red wine vinegar, divided
• 1 medium red onion
• 1 pound fresh mushrooms, such as cremini, shiitake, or oyster mushrooms, or a combination
• 1 small bunch fresh rosemary
• 1/4 teaspoon freshly ground black pepper, divided
• 1 tablespoon Dijon mustard
• 4 ounces Brie cheese

Instructions

Make the crust:

1. Place 1 cup whole-wheat flour, 3/4 cup all-purpose flour, and 1 teaspoon kosher salt in a large bowl and whisk to combine. Add 1/3 cup of the olive oil and mix with a fork until crumbly. Add 1/3 cup water and 1 teaspoon of the red wine vinegar and continue to mix until a soft dough ball forms. Wrap the dough tightly in plastic wrap and refrigerate while you prepare the filling, at least 45 minutes.

Make the filling:

1. Halve and thinly slice 1 medium red onion. Tear 1 pound fresh mushrooms into bite-sized pieces. Strip the leaves from 1 small bunch rosemary sprigs and coarsely chop to get 1 tablespoon.
2. Heat 1 tablespoon of the olive oil in a large skillet over medium heat until shimmering. Add the onion and sauté until softened and golden, 5 to 7 minutes. Transfer to a medium bowl.
3. Add 1 tablespoon of the olive oil to the pan and add half the mushrooms in a single layer. Cook undisturbed until browned on the bottom, about 5 minutes. Season with 1/4 teaspoon of the kosher salt and 1/8 teaspoon of the black pepper and cook, stirring once or twice, until the mushrooms are browned all over and tender, 2 to 3 minutes more. Transfer to the bowl with the onion.
4. Add the remaining 1 tablespoon olive oil to the pan and repeat cooking the remaining mushrooms with the remaining 1/4 teaspoon kosher salt and 1/8 teaspoon black pepper. About a minute before the mushrooms are ready, add about three quarters of the rosemary (reserve the remaining for garnish) and sauté until fragrant, about 1 minute.
5. Remove from the heat. Add the remaining 1 teaspoon red wine vinegar and stir to combine. Transfer to the bowl of mushrooms and onion and stir to combine. Let cool while the dough finishes chilling.

Assemble and bake the galette:

1. Arrange a rack in the middle of the oven and heat the oven to 400°F. Tear a piece of parchment paper, roughly the size of a large rimmed baking sheet, and place it on a work surface. Lightly dust the parchment paper with flour.
2. Unwrap the dough and place it on the parchment. Sprinkle the dough with flour and roll into a round about 12-inches wide. Transfer the parchment paper with the rolled-out dough on it to a rimmed baking sheet.
3. Spread 1 tablespoon 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. Spoon the mushroom mixture evenly over the mustard. Tear 4 ounces Brie cheese with its rind into bite-sized pieces and scatter on top of the filling. Gently fold the edges of the dough over the filling, pleating it about every 2 inches.
4. Bake until the crust is firm and golden-brown, and the cheese is melted and browned in spots, about 30 minutes. Let the galette cool for at least 5 to 10 minutes before serving. Garnish with the reserved rosemary and serve warm, cut into wedges.