Ingredients
- For the dough:
- * 1 1/2 cups all-purpose flour
- * 8 tablespoons (1 stick) unsalted butter, room temperature
- * 1/2 teaspoon kosher salt
- * 5 tablespoons ice water
- For the filling:
- * 3 tablespoons unsalted butter
- * 2 medium white onions, thinly sliced
- * 2 medium leeks, sliced lengthwise and thinly sliced (white part only)
- * 1 teaspoon finely chopped fresh thyme
- * 2 teaspoons finely chopped fresh marjoram
- * 1 tablespoon finely chopped fresh Italian parsley
- * 1/2 cup crème fraîche
- * 1 large egg yolk
Directions
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For the dough:
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1. Cut the butter into small cubes, and combine the flour and butter in a mixing bowl. Using a pastry blender, two table knives, or your hands, cut the butter into the flour until the butter is in pea-sized pieces and the mixture is slightly yellow in color, about 5 minutes.
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2. Mix together the salt and ice water; then add the ice water to the dough and mix until the dough just comes together and can be formed into a ball. Shape the dough into a flat disk, wrap in plastic wrap, and refrigerate for about 30 minutes.
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3. Heat oven to 425°F. Lightly flour a work surface, and roll the dough out into a round approximately 11 inches in diameter and 1/8 inch thick. Cut the dough into a 10-inch round (use an overturned dinner plate and a paring knife to do this). Lay the dough on a baking sheet and evenly spread the filling (see below) while leaving a 1-inch border of dough. Fold the perimeter of the dough (the outer 1 inch) over the filling, and flute the edge by pinching it between your two forefingers.
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4. Make an egg wash by whisking the egg yolk together with 1 teaspoon cold water. Brush the outer edge of the tart with the wash. Place on the middle rack of the preheated oven and bake until the crust is golden, about 20 to 25 minutes.
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For the filling:
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1. Melt 1 tablespoon of the butter in a large frying pan over medium heat. When the butter is foaming, add the onions and leeks (do this in three batches and make sure not to overcrowd the pan). Let the onions and leeks cook until they are caramelized and golden brown, about 10 minutes. Remove from the pan and reserve. Repeat until all the leeks and onions are caramelized.
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2. Combine the herbs and crème fraîche with the onions-and-leek mixture, and let rest until it is cooled, about 5 minutes. Fill the tart and bake immediately.