Heat the butter and oil in a large skillet until very hot. Season with salt and pepper the chops and add them to the skillet. Sear then well on both. Reduce the heat and cook until chops are slightly underdone and still juicy.
Remove them from the skillet, and place them in a ovenproof dish in the warm oven while making the sauce.
Add the onions to the butter and oil remaining in the skillet and cook, covered, over low heat until tender, about 5 minutes. Make sure the onions do not burn. Add the sherry, stock and marmalade to the skillet and bring to a brisk boil, stirring up any brown bits in the skillet.
When the mixture is reduced by one-third, whisk in the heavy cream or the Crème Fraiche and simmer another 2 minutes.
Taste and correct the seasoning if needed. Add the chops to the sauce for 1 minute.
Transfer the chops to a warm dish and spoon some of the sauce over them.