Mix the lamb, onions, rice, pine nuts, salt, pepper and mint into a bowl until evenly combined. Gently open up a grape leaf and place rib-side down onto your work surface. Place a rounded tablespoon of the meat mixture into the centre of the grape leaf. Fold the bottom of the leaf over the meat, fold in the sides and roll into a tight cylinder. Place the rolled grape leaf into a large frypan seam-side down. Repeat with remaining grape leaves, packing them in a tight, single layer.
Pour the water into the frypan and bring to a simmer. Reduce heat to medium-low, cover and simmer until the rice is tender, 50 to 55 minutes. Check occasionally and add more water if needed. Pour off water before serving.