Remove fat from lamb. If the piece is thick, cut it in half lengthwise so that each piece is about 2 cm thick. Mix red wine, vinegar, rosemary and garlic together in a small bowl. Poke several holes at varying intervals in the lamb. Place it and the sauce in a zipper-lock bag to marinate for 20 minutes. Turn after 10 minutes. Heat a medium-size non-stick pan. Remove lamb from marinade and pat dry with a paper towel. Reserve marinade. Brown the meat in the hot pan, and saute 3 minutes per side. Remove to a plate and cover with foil to deep warm. Add reserve marinade to pan, making sure it comes to a boil. Scrape up brown bits in bottom of pan and add mint jelly. Heat to serve with sauce spooned over top.