Asparagus Custard Tart

Blend 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.

Wash leek in a bowl of cold water, agitating water, then lift out leek and pat dry.

Cook 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.

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 sliced asparagus and 1/4 teaspoon pepper and remove from heat.

Put oven rack in middle position and preheat oven to 375°F.

Line a baking sheet with parchment paper and put flan ring in center of baking sheet.

Roll out dough on a lightly floured surface with a floured rolling pin into an 11-inch round, then fit dough inside flan ring, 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, and remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Lightly beat remaining egg with water in a small bowl and brush tart shell all over with some egg wash. Spoon asparagus mixture 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 asparagus tips 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.

*Available at cookware stores and Bridge Kitchenware (212-688-4220; bridgekitchenware.com).

• Pastry dough can be chilled up to 1 day.

Let stand at room temperature until slightly softened before rolling out.• If you don't have a flan ring, you can use a 9- to 9 1/4-inch tart pan with a removable bottom instead. Put a baking sheet in oven while preheating, then bake tart on sheet.• Tart can be baked 2 hours ahead and cooled completely, then kept, loosely covered with plastic wrap, at room temperature. If desired, reheat in a preheated 350°F oven until warm, 15 to 20 minutes.