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1
In a food processor, pulse the flour, salt, and sugar until combined. Scatter the pieces of butter over the flour mixture, add a tablespoon of ice water, and process until it forms a dough, adding the other tablespoons of water as needed. Take the dough out from the food processor bowl, roll into a ball, and pat down into a 5-inch disk. Wrap the dough in plastic wrap and refrigerate for at least thirty minutes while you start the filling of the tart.
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2
In a large skillet, heat the olive oil and melt the butter. Slowly cook the onions over medium-low heat, stirring frequently, until the onions have caramelized (about 30-40 minutes). Add the thyme, stir to combine, and remove from the heat.
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3
Place a baking stone in the oven on the middle rack and preheat the oven to 425F.
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4
After dough has chilled, remove the disk from the refrigerator and roll out into a 12-inch circle. Gently roll up your dough around the rolling pin and unroll over a 9-inch tart pan. Press the dough into the edge of the tart pan and press the rolling pan over the top edge to cut off the extra dough. Freeze the dough in the pan for about 10 minutes. With a ford, prick all over the bottom of the dough to keep it from puffing up, or weight the dough with dried beans in some foil or your pie weight of choice. Bake on the heated baking stone for about 15 minutes, or until the dough begins to br
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5
Remove the dough from the oven and spread the caramelized onions on the bottom. Top with the crumbled and grated cheeses, then place the tomato halves around the tart, cut side down. Return the tart to the oven and bake for about 20-25 minutes, or until the tomatoes are bubbly and the cheese is beginning to brown. Let sit for about 8 minutes before cutting and serving.
