Saute onion in butter until tender (5 minutes). Cool slightly. In large bowl, toss onion with 1/2 cup cheese and remaining ingredients. Press firmly with back of spoon into bottom and sides of well-buttered 9-inch pie plate. Bake in a preheated 450 F oven until lightly browned (20-25 minutes). Invert onto parchment paper-lined baking sheet. Sprinkle with remaining cheese. Preheat broiler; broil 6 inches from heat source until golden brown and cheese melts (1-2 minutes). Carefully remove from parchment paper and transfer to heated serving platter. Cut into wedges. Garnish with parsley. Serve immediately.