Add two tablespoons of the honey to a large plastic bag, add the wine, bay, rosemary and garlic and a generous sprinkle of salt and pepper and shake the bag to mix. Make small incisions with a knife at intervals over the lamb, add to the bag with the onions then seal the bag tightly and put the bag into a roasting tin. Chill for three hours or overnight in the fridge, turning the bag once or twice so that the honey mix flavours both side of the joint.
Take the lamb out of the fridge one hour before cooking to bring it to room temperature. Lift the bag out of the roasting tin, add the potatoes to the tin then the onions from the marinade, pour the honey mix over and arrange the lamb on top. Pour the stock into the tin, cover with foil and twist over the edges of the tin. Roast in a pre-heated oven 160°C / 325°F / Gas Mark 3 for 2 ½ hours.
Remove the foil from the pan, spoon the cooking juices over the lamb, onions and potatoes then drizzle the lamb with the remaining honey. Cook uncovered for 30 minutes until the lamb is a deep golden and the meat is almost falling off the bone.
When ready to serve, mix all the salsa verde ingredients together and spoon into a serving bowl. Carve the lamb into thick slices and add to a serving plate or shallow bowl with the potatoes, onions and cooking juices and a spoonful of salsa verde.