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1
Add the flour and the salt to the bowl of a food processor. (The thyme too, if you're using it.) Cut 1.5 sticks of the butter into slices, and add that too-the butter should be cold, frozen even-keeping everything as cold as possible will yield a flaky crust. Turn the machine on low, and slowly add the ice water, just until the dough comes together.
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2
Form the dough into a disk. Scatter a fine dusting of flour onto some plastic wrap, and wrap up the dough. Move it to the coldest part of the fridge for at least 30 minutes. (You can also do this the night before.)
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3
Roll out the dough; you want it about 1/4-inch thick. I like to roll my dough between two, floured sheets of plastic wrap; I find it's easy, and it minimizes mess. Now ease the dough into a 13-inch tart pan. Move the pan to the freezer for about 20 minutes.
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4
Heat oven to 375F. Blind bake the tart for 20 minutes (I use dried beans). With a fork, poke holes into the base of the tart (this will help prevent bubbles in the dough), and move the tart back into the oven for another 15 minutes, or until the edges are golden.
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5
Quarter the leeks, slice up the white and light green bits, and wash well. Warm the remaining 1 tablespoon of butter in a pan over medium heat, until it is bubbling gently. Add the sliced leeks, and saute until they are wilty, about 7 minutes. Set the leeks aside.
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6
Prep the spring onions: cut away and discard the whiskery bottoms. Wash the stalks well, and then slice.
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7
Wash your chives. I use kitchen shears to mince them. Don't worry about making them perfectly uniform.
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8
Add the: goat cheese, lemon zest, creme fraiche, heavy cream, and tarragon to a large mixing bowl. Use a whisk to blend everything together. Add the eggs, and the remaining 1/2 teaspoon of salt, and mix to incorporate. Lastly, whisk in the minced chives.
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9
Now put the tart together: scatter the leeks, the spring onions, and the aged gruyere across the bottom of the tart. Pour in the cream mixture. Grate some black pepper across the top. Move the tart to the oven, and cook for 22 more minutes, until the cream mixture is just set. Let the tart cool slightly. Serve warm or at room temperature.
