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1
To make the tart dough, put the flour, salt, and freshly ground black pepper into the bowl of your food processor. Add the butter and pulse until the mixture resembles coarse meal. Add the water and pulse until the dough just comes together. Shape the dough into a flat disk, wrap it, and refrigerate it for at least 1 hour (or overnight).
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2
Set a small pot over high heat and add the vegetable stock along with the dried porcini. When the stock begins to boil, lower the heat to maintain a gentle simmer. Cook until the liquid in the pot is nearly fully evaporated, about 15 minutes. Add the heavy cream and continue to simmer until the liquid has reduced down to about 2/3 of a cup (this will take about 10 to 15 minutes). Anything between 1/2 cup and 3/4 cup of liquid is fine.
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3
When the cream has reduced an appropriate amount, remove the pot from the heat. Strain out the mushrooms, reserving the liquid. Dice the mushrooms along with the clove of garlic. Return the diced mushrooms and garlic to the liquid. Set aside. You can make this porcini cream one day ahead of time and store it in the refrigerator.
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4
Set the oven to 425u00b0F. On a well-floured surface, roll out the tart dough. Line a 10-inch fluted removable-bottom tart pan with the dough. Remove and discard any excess dough from the edge of the tart pan. Keep the tart pan in the refrigerator while you prepare the potatoes and celery root.
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5
Peel the potatoes and slice them as thin as you can, using a mandolin or a knife. Slice the celery root as thin as you can too. In a large mixing bowl, toss the sliced potato and celery root with the porcini cream. Season with salt and pepper.
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6
Add the potato and celery root mixture to the tart pan in even layers. Scrape any excess liquid over the top of the tart. Bake at 425u00b0F for 10 minutes, then reduce the heat to 375u00b0F and cook for an additional 45 to 55 minutes, until the crust looks lightly golden and the potatoes on the top of the tart are beginning to get crispy and brown.
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7
Serve the tart warm, with a drizzle of arugula pesto to garnish. See below for arugula pesto recipe. Enjoy!
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8
Place half the arugula and half the olive oil into a food processor. Pulse it a few times. Add the rest of the arugula and the rest of the olive oil and pulse until it looks like pesto. Add the parmigiano, lemon juice, and two pinches of salt. Pulse a few times. Taste the pesto. Think about what makes you happy, and then add more salt and/or lemon juice as necessary.
