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1
Make the crust - Measure the flour into a large bowl, cut the butter into small pieces, then add to the flour.
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2
Using your hands, work the butter into the flour until the pieces are small (about the size of a pea) and the mixture has a grainy texture. Stir in the parmesan cheese.
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3
Add the water (very cold) a little bit at a time, mixing until the dough is all incorporated into a ball. If 1 oz of water doesn't seem to be enough, you can add a bit more, but add it a little bit at a time, and use the least amount of water possible to hold the dough together.
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4
Flatten the dough into a disc, wrap in plastic and refrigerate for at least 15-20 minutes before working with it further.
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5
Preheat oven to 400 degrees.
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6
Once the dough has been chilled, roll it out on a well floured surface and shape it into a greased 9-inch pie or tart pan.
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7
Cover crust with aluminum foil, and add some dry beans or rice to weight it down. Bake at 400 degrees for 10 minutes, then remove the foil and weights and bake uncovered an additional 10 minutes. Remove from oven and let cool while you prepare the filling. Reduce oven heat to 350 degrees.
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8
Heat oil over medium heat in a pan. Add the onion, and cook until it begins to soften, about 3-4 minutes.
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9
Add mushrooms, thyme, 1/2 tsp salt, and 1/2 tsp pepper, and continue cooking, stirring often, until mushrooms are cooked and beginning to brown.
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10
In a medium bowl, whisk together the eggs, milk, 1/4 tsp salt, and 1/4 tsp pepper. Whisk in the mascarpone cheese. Stir in the mushroom mixture and 1/2 cup grated fontina cheese.
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11
Pour into prepared crust, top with remaining 1/2 cup fontina cheese, and bake at 350 degrees until filling is golden, well set, and slightly puffed in the center, about 30 minutes. Serve tart warm or at room temperature.
