Rub the oil in a roasting pan. Sprinkle the pork with salt and pepper and rub the pork all over with the mustard. Set it in the pan and pour in 1/2 cup/125 ml water. Roast until the pork is tender, about 1 hour 30 minutes. (If the water evaporates in the pan, add a little more.)
Remove the pork to a serving dish and keep warm. Fry the shallot in the roasting pan on the stovetop. Deglaze with wine and boil to reduce by half. Stir in the creme fraiche and rosemary, and reduce to sauce consistency. Check the seasonings. Slice the pork, pour the sauce on top and serve.