1. To make crust: Sift flour and salt into large bowl. Rub in butter with fingertips until mixture resembles coarse meal. Beat 1 egg and 2 Tbs. water in small bowl. Stir egg mixture into flour mixture just until dough comes together, adding 1 Tbs. more water if necessary. Flatten dough into disk, wrap in plastic wrap, and chill 1 hour, or overnight.
2. Preheat oven to 350°F. Roll out dough to 12-inch circle on floured work surface. Press dough into 9-inch springform pan or fluted tart pan with removable bottom. Trim edges, and prick bottom all over with fork. Line tart shell with parchment paper, and fill with dried beans. Bake 25 minutes, or until barely golden. Remove beans and paper. Beat remaining egg, and brush on bottom of crust. Bake 5 minutes more.
3. To make Filling: Cook asparagus in large pot of boiling salted water 3 minutes. Drain, and cut into 1 1/4-inch lengths.
4. Heat oil in skillet over medium-low heat. Add onions, and cook 8 to 10 minutes, or until softened. Remove from heat.
5. Whisk together eggs and milk in bowl. Stir in onions and asparagus. Pour into Shortcrust, and place pan on baking sheet. Sprinkle tart with Parmesan. Bake at 350 degrees for 50 minutes to 1 hour, or until tart is set in center, covering edges with foil if they start to get too brown. Unmold, and serve.