1 1/2 cups (191 grams) all-purpose flour
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1 tablespoon granulated sugar
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3/4 teaspoon fine salt
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10 tablespoons (142 grams) cold unsalted butter
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4 to 6 tablespoons ice cold water
1 tablespoon extra virgin olive oil
1 lb (16 oz) cremini (Baby Bella) mushrooms stemmed and thinly sliced
½ lb (8 oz) shiitake mushrooms stemmed and thinly sliced
1 tablespoon unsalted butter
½ large yellow onion finely julienned
2 large garlic finely minced
4-5 sprigs fresh thyme plus more for garnishing
1 sprig fresh rosemary
½ cup dry sherry
freshly ground black pepper
1½ oz aged blue cheese (or 3 oz aged goat cheese) crumbed (such as Point Reyes or Stilton)
1. Freeze your butter until hardened, then use the grater attachment of your food processor to shred it. If you don't have the grater attachment, use a bench scraper to cube cold but not frozen butter into 1/2-inch pieces. Freeze the cubed butter for 10 minutes.
2. In the bowl of a food processor, add the flour, sugar, and salt. Pulse a few times to combine. Add the cold grated or cubed butter and pulse for about 30 seconds, or until the butter pieces are uniform in size.
3. Drizzle a few tablespoons of water on the mixture and pulse briefly a few times to moisten. Continue adding the remaining water until the dough sticks together and stays stuck when pressed between two fingers. It’s okay if the dough is still crumbly and messy.
4. Turn the mixture out onto a clean work surface. Use your hands to press the dough together into a craggily mass. Gently fold one half of the dough over and onto itself, repeating this three times, until it’s come together into a cohesive mass.
5. Use a bench scraper to cut the dough in half (if making double crust). Wrap each half in plastic. Press down each plastic-wrapped dough to create a 4-inch wide disk shape. Refrigerate at least six hours, preferably overnight.
Prepare the Mushroom Filling: Heat a 12-inch stainless steel skillet over medium-high heat. Add the olive oil, followed by the mushrooms, spreading them evenly across the pan. Sauté the mushrooms, stirring every minute or so, until they've released their moisture, shrink down considerably, and begin to caramelize, about 4 to 6 minutes. Season lightly with salt and pepper.
Add the butter and allow it to melt slightly. Add the julienned onion, thyme, and rosemary sprigs. Cook, stirring frequently, until softened, about 3 to 5 minutes. Add the garic and cook for an additional minute or until fragrant.
Deglaze the pan with sherry, scraping any brown bits that may have formed on the bottom of the pan, and continue to cook until the liquid has cooked off and the pan is almost dry. Season mixture with salt and pepper (*be mindful of salt, as we will be adding cheese to the mushroom mixture).
Make the Egg Wash: Whisk together the egg and milk in a small bowl. Set aside.
Assemble and Bake the Galette: As the mushroom mixture is cool, preheat the oven to 400°F (205°C) with a rack in the center position. Line a half sheet pan with parchment paper and set aside.
Remove the dough from the fridge and lightly dust with flour. If it is too cold, allow it to sit at room temprature for 5 to 10 minutes before proceeding. On a floured work surface, roll into a 12-inch round. Transfer to the lined baking sheet. Note: If the dough warms too quickly or becomes sticky, chill in the refrigerator for 10 to 15 minutes before proceeding.
Crumble the blue cheese (or goat cheese) into a large bowl, add the cooled mushroom mixture (discard the thyme and rosemary sprigs), and stir together gently.
Spoon the mixture into the center of the dough, spreading it in a thick even layer, leaving a 1½ inch border on all sides. Fold and pleat the edges of the dough border over the mushrooms – leaving the center exposed. Lightly brush the edges of the dough with egg wash.
Bake for 40 to 50 minutes, rotating the pan halfway, or until the crust is golden brown in color. Place on cooling rack and allow to cool for 5 to 10 minutes before slicing and serving. Garnish with fresh thyme leaves, as desired. Serve hot, warm, or at room temperature.