Make pastry.
Mix 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.
Turn 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.
Make filling while pastry chills.
Wash leek in a bowl of cold water, agitating water, then lift out leek and pat dry.
Cook asparagus in a 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. Set 6 or so pieces of the asparagus aside for decoration, if you choose to.
Heat 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 remainder of sliced asparagus and 1/4 teaspoon pepper and quickly remove from heat.
Finish filling and assemble and bake tart.
Put oven rack in middle position and preheat oven to 375°F.
Roll out dough on a lightly floured surface with a floured rolling pin into an 11-inch round, then fit dough inside 10" tart pan, pressing dough against bottom edge and side of ring. Run rolling pin over top edge of ring to cut off excess dough.
Whisk together cream, 2 eggs, tarragon, leeks and remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Spoon asparagus into shell, spreading evenly, then pour cream mixture over asparagus.
Bake tart until filling is just beginning to set but still loose on top, 20 to 25 minutes. Scatter 6 or so asparagus tips set aside earlier over top, pressing lightly if necessary to help them settle into filling, then continue to bake until custard is golden and just set but still slightly wobbly in center, about 30 minutes more (custard will continue to set as it cools).
Cool tart on baking sheet on a rack until warm, about 30 minutes. Loosen edge with a small sharp knife, then lift off flan ring. Serve warm or at room temperature, cut into wedges.
