Step 1

Prepare Dough: Whisk together the all purpose flour and salt in medium mixing bowl. Add the cubed butter and toss in the dry flour mixture, separating any pieces that stick together, until lightly coated. Use a pastry cutter to cut the butter into the flour until the mixture resembles coarse meal and butter pieces are no bigger than the size of small peas.

Step 2

In a seperate bowl, whisk together the sour cream, lemon juice, and ice water. Create a well in the flour mixture and add the wet ingredients. Stir with a spatula until the dough begins to clump together. Transfer the dough and any dry bits to a lightly floured countertop. Use your hands to press and knead the dough gently until it comes together. Flatten into a thick disk, cover tightly with plastic wrap, and refrigerate for a minimum of 1 to 2 hours - or until chilled and firm.

Step 3

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.

Step 4

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.

Step 5

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).

Step 6

Transfer the mushroom mixture to a sheet pan to cool, spreading it into an even layer. Allow mixture to come to room temperature before assembling the galette.

Step 7

Make the Egg Wash: Whisk together the egg and milk in a small bowl. Set aside.

Step 8

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.

Step 9

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.

Step 10

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.

Step 11

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.

Step 12

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.