• Preheat your oven to 400F with a rack in the middle position. Line a baking sheet with parchment paper.

Start by making the Crust Dough

• Mix together the flour and salt in a large bowl. Cut the cold butter into the flour mixture with a pastry blender or with clean hands, until having a fine crumb mixture.
• Make a well in the center of the flour butter mixture. Place the sour cream, water and vinegar in the center. Mix the wet ingredients together and then bring in the flour mixture to the center and folding until just combined. Do not over mix to prevent a tough dough.
• Bring the dough together with your hands to form a small disk. Wrap the dough tightly in plastic wrap and place in the refrigerator while you make the filling.

Make the Mushroom filling

• Place a large frying on medium-high heat. Melt the 2 tablespoon of butter with olive oil in the pan. Add the sliced leek and mushrooms with the salt and cook until the mushrooms have released their liquids and they have mostly evaporated.

Make the Cheese Filling

• Stir together the Boursin and the goat cheese with a spoon or rubber spatula until smooth. Add the juice of a lemon (or 2 tbsp) into the cheese and mix it in until well incorporated. Set aside.

Assemble the Galette

• Remove the dough from the plastic wrap and plate it on a floured surface. Roll it out into a circle of 14 to 15 inches in diameter. It helps to flip over the dough and rotating it in quarter turns, while lightly flouring your surface, in order to get an even circle.
• Carefully transfer the rolled dough to the lined baking tray, lining up the center of the dough with the center of the baking tray. The sides of the dough might go over the tray, but they will be folded in later.
• Spread the cheese filling evenly in the center of the dough, leaving a 3 inch border of dough all around.
• Place the mushroom filling on top of the cheese, making sure to stay on the same surface area as the cheese filling.
• Fold over a 4-inch piece of the border dough over the filling. Continue doing so, a section at a time, overlapping on the last section, until all sides are folded in.
• Mix together the egg with a tablespoon of water or milk to make an egg wash. Brush some of the egg wash on the crust. Sprinkle the crust with the Everything Bagel Seasoning.
• Bake the galette in the preheated oven for 15 minutes. Turn the tray around and keep baking for another 10 minutes, or until the dough golden brown.
• Remove from the oven and let rest 5-10 minutes before cutting into it.