Butternut Squash and Caramelized Onion Galette

In a medium bowl, combine 2 1/2 cups flour and 1/2 teaspoon salt. Add 2 whole sticks of butter and use a pastry blender to break up the bits of butter into the flour until it has the texture of cornmeal, with the biggest pieces the size of small pebbles.
In a small bowl, whisk together 1/2 cup sour cream, 1 tablespoon white vinegar, and 1/3 cup ice water. Pour the mixture over the butter-flour mixture (from the first step), and stir together with a wooden spoon until a dough forms, kneading once or twice on the counter if needed to bring together.
Pat the dough into a ball, wrap it in plastic, and chill in the refrigerator for an hour or up to two days.

Preheat the oven to 400 (F).
To prepare squash: Peel the squash, then halve and scoop out seeds. Cut into 1/2-inch to 3/4-inch chunks. Pour 2 tablespoons of olive oil into one or two smaller baking sheets, spreading it to an even slick. Lay the squash chunks on the baking sheet in one layer, sprinkle with 1/2 teaspoon of the salt, and a couple turns of freshly ground black pepper, and roast in the preheated oven for 30 minutes, or until squash is tender, turning the pieces occasionally so that they brown evenly. Set aside to cool slightly. Leave the oven on.
While the squash is roasting, melt 1 tablespoon butter and remaining 1 tablespoon of olive oil in a heavy frying pan, and cook the onions over medium-low heat with the remaining teaspoon of salt, stirring occasionally, until soft and tender, about 25 minutes. Stir in the cayenne pepper.
When the squash and onions are finished, mix them together in a medium bowl along with 2 cups grated fontina cheese and 1 teaspoon chopped fresh thyme.

On a floured work surface, roll the dough out into a 16- to 17-inch round (or two 14- to 16- inch rounds, if making two galettes). Transfer to a parchment-lined baking sheet.
Spread the squash-and-cheese mixture over the dough, leaving a 2 to 2 1/2-inch border. Fold the border over the squash and cheese, pleating the edge to make it fit. The center will be open.
In a small bowl, whisk together 1 egg with 1 teaspoon of water to make egg-wash. Brush the outside of the crust with the wash.
Bake until golden brown, 30 to 40 minutes. Remove the galette from the oven, let stand for five minutes, then slide onto a serving plate. Cut into wedges and serve hot, warm or at room temperature.

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