Fry the onions in the butter in a large skillet over medium-low heat for at least 15 to 20 minutes (maybe longer), stirring occasionally, until the onions are deep golden brown. Set aside to cool.
Fry the bacon until chewy. Chop into large bite-sized pieces and set aside to cool.
Preheat the oven to 400 degrees. Roll out the pie crust and press it into a large fluted deep tart pan (or a deep dish pie pan).
Whip the eggs, cream, salt and pepper in a large bowl, then mix in the onions, bacon, and cheese. Pour the mixture into the pie crust.
Place the pan on a rimmed baking sheet, cover lightly with aluminum foil, and bake it for about 40 to 45 minutes. Remove the foil and continue baking for 10 to 15 minutes, or until the quiche is set and the crust is golden brown. (The quiche will still seem slightly loose, but will continue to set once remove from the oven.)
Remove from the oven and allow to sit for 10 to 15 minutes. If using a tart pan, remove the quiche from the pan, cut into slices with a sharp serrated knife, and serve!