Asparagus Custard Tart

Name: Asparagus Custard Tart
Ingredients: 1 cup all-purpose flour; 1/2 teaspoon salt; 3/4 stick (6 tablespoons) cold unsalted butter, cut into 1/2-inch cubes; 1 large egg, lightly beaten; 1 medium leek (white and pale green parts only), quartered lengthwise, then cut crosswise into 1/3-inch pieces; 1 lb medium asparagus, trimmed; 1 tablespoon unsalted butter; 1/2 teaspoon black pepper; 1 1/3 cups heavy cream; 3 large eggs; 2 teaspoons finely chopped fresh tarragon; 2 teaspoons water; parchment paper; a 9 1/4-inch flan ring* (see cooks' note, below)