Preheat oven to 160°C. Place lamb in a deep roasting pan and rub all over with harissa. Add chicken stock, onion, garlic and lemon to the pan.
Roast for 4 hours or until lamb is very tender. Cool, then shred lamb into pieces, discarding bone.
Spread the mayonnaise inside the buns, adding extra harissa, then fill with lamb, mint leaves and snow pea sprouts. Serve warm.