Roll pie crust from center to edges on a lightly floured surface forming a circle about 12 inches in diameter. Ease pastry into a 9-inch tart pan with a removable bottom, pressing dough up into fluted sides of tart pan. Trim edges, reserving scraps. Do not prick pastry. Line pastry with a double thickness of foil. Bake in a 450 degrees F oven for 8 minutes. Remove foil. Bake 4 minutes more or until crust is dry and set. Reduce oven temperature to 375 degrees F.
For filling, melt butter in a medium skillet; stir in brown sugar and vinegar. Add onions. Cook, uncovered, over medium-low heat for 10 to 12 minutes or until onions are tender and light brown, stirring occasionally.
Beat cheese, eggs, chervil or marjoram, and pepper in a mixing bowl with an electric mixer on low speed until combined (cheese will still be lumpy). By hand, stir in onion mixture, milk, wine, and parsley. Ladle filling evenly into baked tart shell.
Bake tart in a 375 degrees oven about 20 minutes or until a knife inserted near center of filling comes out clean and pastry is golden. Cool 15 minutes in pan on a wire rack.
Meanwhile, roll out pie crust scraps to 1/8-inch thickness; cut into decorative shapes with small cutters. Place on ungreased baking sheet. Brush lightly with a mixture of egg yolk and 1 teaspoon water. Bake cutouts in 375 degrees F oven for 6 to 7 minutes or until golden. Carefully remove sides of tart pan. Decorate top with baked cutouts and add green onion curls, if desired. Cut tart into wedges. Serve while warm. Makes 12 appetizers.