Preheat oven to 350°. Line a 9-inch tart pan with 1 piecrust; prick bottom of crust with fork. Line sides with foil, add pie weights (or dried beans), and place on bottom rack of oven. Bake at 350° for 18 minutes or until golden brown.
Melt butter in a large sauté pan over medium heat. Sauté onion 10 minutes or until soft and translucent. Place onion in large bowl, and let cool. Add egg and next 4 ingredients; fold together, and pour mixture into crust. Top with bacon, and bake at 350° for 20 minutes or just until set. Let stand for 10 minutes before serving.