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1
Make the dough: Whisk together the flour and salt in a large bowl. Sprinkle bits of butter over the dough and using a pastry blender, cut it in until the mixture resembles coarse meal, with the biggest pieces of butter the size of tiny peas. In a small bowl, whisk together the sour cream, lemon juice and water and add this to the butter-flour mixture. With your fingertips or a wooden spoon, mix in the liquid until large lumps form. Pat the lumps into a ball; do not overwork the dough. Cover with plastic wrap and refrigerate for 1 hour.
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2
Make the filling: In a large saucepan, heat 1 tablespoon of the olive oil over medium high heat. Add the onion saute until almost soft, about 5 minutes. Add the spinach and continue cooking until the spinach wilts, about 2 minutes. In a small bowl, whisk another tablespoon of olive oil and the minced garlic together; set aside. In a separate bowl, mix the ricotta, goat cheese, and 1 teaspoon of the garlicky olive oil together and season with salt and pepper to taste. Add in the onion and spinach mixture.
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3
Prepare the galette: Preheat oven to 400u00b0F. On a floured work surface, roll the dough out into a 12-inch round. Transfer to an ungreased baking sheet (though if you line it with parchment paper or a Silpat, it will be easier to transfer it to a plate later). Spread the cheese and onion mixture evenly over the bottom of the galette dough, leaving a 2-inch border. Shingle the mushrooms attractively on top of the ricotta in concentric circles, starting at the outside edge. Drizzle the remaining tablespoon of the garlic and olive oil mixture evenly over the mushrooms. Fold the border over the filling, pleating the edge to make it fit. The center will be open. Brush the crust with the egg yolk glaze.
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4
Bake the galette until the cheese is puffed, the mushrooms are slightly wilted and the galette is golden brown, 30 to 40 minutes. Remove from the oven let stand for 5 minutes, then slide the galette onto a serving plate. Cut into wedges and serve hot, warm or at room temperature.
