Have butcher slice meat into eight 2-ounce slices, or freeze it briefly and slice it yourself with a sharp knife. Place slices between 2 sheets of lightly oiled waxed paper and pound with a mallet or bottom of a skillet until they are paper-thin. Soak porcini mushrooms in hot water to cover for 1 hour.

Place beef slices on a flat surface. Divide prosciutto slices evenly among them and sprinkle chopped celery leaves over prosciutto. Roll each beef slice into a neat bundle and tie securely with kitchen string.

Heat 3 tablespoons oil in a heavy skillet over medium-high heat until a light haze forms. Brown beef rolls on all sides, in batches if necessary, then set aside. If necessary, add a little more oil to coat surface of pan, then add garlic and onion and saute over moderate heat until softened but not browned.

With a slotted spoon, remove mushrooms from soaking liquid and squeeze dry. Stain liquid through cheesecloth to remove any grit or sand. Add mushrooms, tomatoes, and reserved soaking liquid to skillet. Season with salt and pepper to taste. Return meat to skillet and bring sauce to a simmer. Cover, reduce heat to maintain a slow simmer, and cook 2 hours. Check meat after 1 hour, adding a little water if sauce has reduced too much. When meat is quite tender, transfer rolls to a warm serving plate, cut strings carefully with a small knife, and cover with sauce.

Food Processor Version: Use metal blade to process 3 to 4 cloves of garlic for 10 seconds. Cut 1 large onion into eighths and add to work bowl. Turn motor on and off 3 or 4 times. Scrape bowl and

pulse 2 to 3 more times until onion is minced.