Asparagus Custard Tart

• 1 cup all-purpose flourflour
• 1/2 teaspoon saltsalt
• 3/4 stick (6 tablespoons) cold unsalted butter, cut into 1/2-inch cubesbutter
• 1 large egg, lightly beatenegg
• 1 medium leek (white and pale green parts only), quartered lengthwise, then cut crosswise into 1/3-inch piecesleek
• 1 lb medium asparagus, trimmedasparagus
• 1 tablespoon unsalted butterbutter
• 1/2 teaspoon saltsalt
• 1/2 teaspoon black pepperpepper
• 1 1/3 cups heavy creamheavy-cream
• 3 large eggsegg
• 2 teaspoons finely chopped fresh tarragontarragon
• 2 teaspoons waterwater
• parchment paper; a 9 1/4-inch flan ring* (see cooks' note, below)

1. 01Blend together flour, salt, and butter in a bowl with your fingertips or a pastry blender (or pulse in a food processor) until most of mixture resembles coarse meal with some small (roughly pea-size) butter lumps. Drizzle evenly with egg and gently stir with a fork (or pulse in processor) just until a dough forms.
2. 02Turn out dough onto a lightly floured surface and divide into 4 portions. With heel of your hand, smear each portion once or twice in a forward motion to help distribute fat. Gather dough into a ball, then flatten into a 5-inch disk. Chill dough, wrapped in plastic wrap, until firm, at least 30 minutes.
3. 03Wash leek in a bowl of cold water, agitating water, then lift out leek and pat dry.
4. 04Cook asparagus in a wide 4- to 5-quart pot of boiling salted water , uncovered, until just tender, about 5 minutes. Transfer asparagus with tongs to a bowl of ice and cold water to stop cooking, then drain and pat dry. Cut off and reserve tips (leave more stalk if asparagus is thin), then thinly slice stalks crosswise.
5. 05Heat butter in a 10- to 12-inch heavy skillet over moderately low heat until foam subsides, then cook leek with 1/4 teaspoon salt, stirring, until softened, 6 to 8 minutes. Stir in sliced asparagus and 1/4 teaspoon pepper and remove from heat.
6. 06

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