In large bowl, with fork, stir flour and 1 teaspoon salt. With pastry blender or two knives used scissor-fashion, cut in shortening and 1/2 cup cold margarine or butter until mixture resembles coarse crumbs.
Sprinkle 7 to 8 tablespoons cold water, a tablespoon at a time, into flour mixture, mixing with fork after each addition until mixture is just moist enough to hold together. With hands, shape dough into a ball.
On floured surface, with floured rolling pin, roll dough into a rectangle approximately 18-inch by 13-inch. Gently fold rectangle into fourths and carefully lift into ungreased 15 1/2-inch by 10 1/2-inch jelly-roll pan; unfold.
Lightly press dough onto bottom and sides of pan. Fold overhang under and pinch to form decorative edge level with rim of pan. Wrap and refrigerate tart shell about 30 minutes.
Meanwhile, in deep 12-inch skillet, melt remaining 3 tablespoons margarine or butter over medium-high heat. Add onions and cook, stirring frequently, until onions are golden brown, about 25 minutes. Remove from heat and cool to room temperature.
Preheat oven to 425 degrees F. Line tart shell with foil and fill with pie weights, uncooked rice, or dried beans. Bake tart shell 20 minutes; remove foil and weights and bake 10 minutes longer or until golden. Remove tart shell from oven and turn oven control to 400 degrees F.
In bowl, with wire whisk or fork, beat eggs, milk, Parmesan, thyme, parsley, pepper, and 1 teaspoon salt.
Spread cooled onions over tart shell. Pour egg mixture over onions.
Bake tart 25 minutes or until egg mixture is set and tart is nicely browned. Serve hot. Or cool on wire rack; wrap and refrigerate to serve cold later.