Wash the meat and pat dry. Make a long cut along the long side to create a deep pocket but leave about 2cm uncut. Rub on all sides with salt, pepper, garlic, nutmeg and olive oil. Wrap in cling film and refrigerate for 12 hours.
Preheat oven to 180 C / Gas 4.
Unwrap the meat and place prunes in the pocket and close it. Tie the roast together with kitchen string.
Transfer to an ovenproof dish with a lid and roast, covered, for 1.5 hours.
About 15 to 20 minutes before the end of the roasting time, remove the lid so the top can brown.