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1
To make the crust, in a food processor, add both flours and the salt and pulse to combine. Add the butter and thyme and pulse until pea-size chunks form. Keep pulsing while adding the vinegar and then the cold water, 1 tablespoon at a time, stopping when the dough just barely holds together. Form the dough into a disk, wrap it in plastic wrap, and chill in the fridge for at least 30 minutes or up to overnight.
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2
Preheat the oven to 400u00b0F.
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3
On a lightly floured surface, roll out the dough into a 13-inch circle. It should be about 1/4 inch thick. Roll the dough around the rolling pin and lift it into an 11-inch fluted tart pan with a removable bottom. Press the dough into the edges and up the sides, making sure to patch up any holes. Gently roll your rolling pin across the top of the tart pan to remove the extra dough and create a clean edge. Prick the bottom of the dough with a fork, lay a piece of parchment paper on top, and fill the tart shell with pie weights (I use rocks from the yard-classy, I know). Bake for 15 minutes. Remove the parchment and weights and bake until the top looks almost dry, another 10 to 12 minutes. Remove from the oven and let cool.
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4
While the crust is cooling, prepare the filling. On a rimmed baking sheet, toss the squash with 1/2 tablespoon of the olive oil, 1/2 teaspoon salt, and the nutmeg. Spread in an even layer and bake until the squash begins to brown around the edges, 20 to 25 minutes. Remove from the oven and let cool.
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5
In a large saute pan over medium heat, warm 1 tablespoon of the olive oil and the garlic. When the garlic starts to sizzle a bit and becomes fragrant, add the Swiss chard, red pepper flakes, and a pinch of salt. Saute until the chard is wilted, about 5 minutes. Transfer to a large mixing bowl and set aside.
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6
Peel and halve the onion and thinly slice. In the same pan you used for the chard, heat the remaining 1/2 tablespoon olive oil over medium heat. Add the onion and a pinch of salt and stir every so often until caramelized, about 20 minutes. When the onions are a nice light brown color, add the balsamic vinegar, stir, and turn off the heat. The onions will absorb the vinegar as they cool a bit.
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7
Squeeze out any excess water from the Swiss chard and return to the bowl. In a separate bowl, whisk the eggs until blended well, then add to the chard. To the bowl with the chard, add three-fourths of the squash, half of the cheese, the onion, and a few grinds of black pepper. Gently mix everything together and pour into the tart pan. Spread into an even layer. Scatter the remaining squash and cheese across the top. Bake in the oven until the egg is just set and the top is browned, 24 to 28 minutes. Remove the tart from the oven and allow it to cool for 5 to 10 minutes before cutting into slices and serving.
