Preheat oven to 325 . Place prepared pastry into an 8" pie plate or tar dish. Place a sheet of parchment paper over the crust, then add a layer of pie weights, rice, or dried beans. Blind-bake for 20 minutes, then let cool before removing parchment and weights.
While the crust bakes, heat butter and oil in a large fry pan over medium-low heat. Sauté onions until soft and translucent. Remove from heat and allow to cool.
While the crust and onions cool, prepare the egg mixture. In a large bowl, combine eggs with heavy cream, salt, pepper, and nutmeg, whipping by hand until thoroughly combined. Add cooled onions and stir together. Pour mixture into the pastry shell and bake for 30 minutes. Cool, cut, and serve with a green salad and Dijon mustard.