Snap off tough ends of asparagus (trimmed to about 5 inches). Cook in boiling water to cover 30 seconds; drain. Plunge asparagus into ice water to stop the cooking process; drain. Reserve 9 spears; set aside. Coarsely chop remaining asparagus; set aside.
Unfold piecrusts; stack on a lightly floured surface, and roll into a 14-inch circle. Fit piecrust into an 11-inch tart pan, trimming excess dough. Line piecrust with foil, and fill with pie weights or dried beans. Place tart pan on a baking sheet.
Bake at 425° for 12 minutes; remove weights and foil. Bake 2 more minutes. Cool on a wire rack.
Melt butter in a skillet over medium-high heat. Add onion, and saute 5 minutes or until tender. Set aside.
Brush bottom and sides of crust with mustard. Sprinkle with half of cheese, chopped asparagus, onion, and remaining cheese. Arrange reserved asparagus spears over cheese.
Whisk together half-and-half and remaining ingredients. Pour mixture evenly over asparagus.
Bake at 375° for 25 minutes or until set and golden brown. Let stand 15 minutes.