Make marinade: Process tomato, onion, garlic, vinegar, sugar, and rosemary in a food processor until finely chopped. With the motor running, gradually add wine, processing until well combined.
Place lamb in a large, shallow glass or ceramic dish. Pour over marinade, turning to coat. Cover and refrigerate for 6 hours or overnight, turning the lamb occasionally to marinate evenly.
Preheat oven to 130°C (110°C fan-forced). Transfer lamb and marinade to a roasting pan. Bake for 1 hour.
Season with salt and pepper. Cover tightly with foil. Bake for 2 hours.
Uncover and turn the lamb. Cover tightly with foil again. Bake for a further 2 hours.
Remove foil. Turn lamb. Bake uncovered for a further 1 hour, or until the lamb is very tender.