Preheat oven to 325 degrees. Soak prunes in wine, if you wish, for as long as possible. Drain prunes, reserving wine. With a long sharp knife, make a slit in the center of the loin and stuff it with prunes (reserve prunes that won't fit into pork).
In a large casserole heat butter. When foaming subsides add the onion or leek, celery and carrot. Cover and cook for 5 to 8 minutes or until the vegetables are tender. Remove them with a slotted spoon and reserve for later. Add vegetable oil to the casserole and heat. When hot, add the loin and brown on all sides, including the ends. When pork is browned, remove it to a plate. Discard all fat from the casserole, but leave behind any drippings. Return cooked vegetables to the bottom of the casserole. Add the meat on top and add any prunes which didn't fit into the loin.
Cover loin with reserved white wine you soaked the prunes in, broth, bay leaf and garlic. Put a lid on casserole and set in oven. Cook for 1 and 1/2 to 2 hours or until pork juices run a clear yellow (difference in time depends on how thick the loin is; internal temp is 165 degrees). Remove casserole from oven and remove the pork and let rest while you make the sauce. Strain the juices into a "degreasing" cup or heat-resistant cup. Discard aromatic vegetables and prunes. The fat will rise above the juices; discard it. Return juices to casserole and boil down until thickened. Finish with a tablespoon of butter or not as you wish. Slice pork into thin slices, serve with saffron rice, sugar snap peas and spoon sauce over the top.