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1
Melt 1 tablespoon of the butter in a large frying pan over medium heat.
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2
When it foams, add 1/3 of the onions and leeks.
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3
Season well with salt and freshly ground black pepper.
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4
Cook until onions and leeks are caramelized and golden brown, about 10 minutes.
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5
Season again with salt and pepper.
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6
Remove from the pan and reserve in a mixing bowl.
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7
Cook remaining onions and leeks in two more batches, using 1 tablespoon of the butter per batch.
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8
Combine herbs and creme fraiche with onion-leek mixture and check the seasoning.
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9
Add more salt and pepper as necessary, and let the mixture rest until it has cooled, about 5 minutes.
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10
Heat the oven to 425 degrees F and arrange a rack in the middle.
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11
Lightly flour a work surface and roll the pie dough out into a round approximately 11 inches in diameter and 1/8 inch thick.
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12
Cut the dough into a 10-inch round (an overturned dinner plate and a paring knife work great).
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13
Lay the dough on a baking sheet and evenly spread the filling throughout, leaving a 1-inch border.
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14
Fold the perimeter of the dough (the outer 1 inch) over the filling and flute the edge by pinching it between your forefingers.
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15
(Check out our photo tutorial on for more information.)
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16
Make an egg wash by whisking the egg yolk with the cold water.
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17
Brush the outer edge of the tart with the wash. Place on the middle rack of the preheated oven and bake until the bottom of the crust is golden, about 20 to 25 minutes.
