MAKE THE CRUST: Place the flours and salt into the bowl of a food processor with the blade attached. Process for a couple of seconds to combine. Add the butter and pulse until the flour and butter become crumbs, about 7 to 10 pulses. The crumbs should be uneven, with some fine crumbs and some the size of small pebbles. Add the Parmesan and pulse to combine.
Add the water and pulse for a couple of seconds just to combine. Do not process until the dough becomes a ball.
Remove the dough from the bowl and gather into a ball. With the heel of your hand, push the dough forward onto a board to spread the butter and bind together. Form into a 4-inch flattened disk. Wrap in plastic and chill in he refrigerator for at least 1 hour before rolling out. You can keep the dough refrigerated for up to 3 days or frozen for up to a month.
MATKE THE TART: Warm 2 tablespoons of the olive oil in a medium skillet. Add the onion and thyme sprigs and cook over medium-low heat until the onion is tender and caramelized about 20 minutes. Remove the thyme stems (most of the leaves will be in the onions) and transfer the onions to a medium bowl.
Meanwhile, warm the remaining 2 tablespoons oil in another medium skillet. Add the mushrooms and cook over medium heat until they have released their juices, about 20 minutes. Stir the mushrooms into the onions.
If you don't have syrup, pour the balsamic vinegar into a small skillet and simmer, uncovered, until reduced to 3 tablespoons, about 5 minutes. Keep an eye on it the last couple of minutes so it doesn't burn.
Whisk the egg and the syrup or reduced vinegar together in a small bowl. Stir in the cheese, 1/2 teaspoon salt, and a sprinkling of black pepper. Stir into the mushroom mixture. Preheat the oven to 375°F. Take out the piecrust dough from the refrigerator and let sit at room temperature for at least 15 minutes. Then, either on a lightly floured board or between 2 pieces of parchment paper, roll out the dough, starting from the center and moving outward in all directions. Roll the dough until it is as thin as possible (about 1/16th inch).
Transfer the dough to the tart pan. Lightly push in the crust to meet the contours of the pan and fold over the edge to fit inside. Fill the pan with the mushroom mixture and bake for 30 to 40 minutes, until the crust is golden and pulls from the sides of the pan. Serve warm or at room temperature.
Makes one 9-inch tart to serve 6
Reprinted with permission from © Myra Kornfeld, The Healthy Hedonist: Holidays, published by Simon and Schuster click for book review
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