For the stuffing:
In a bowl, mix the carrots, leeks, celery root, celery leaves, parsley, chives, and chervil.
Cut or lamb roast and lay flat. Stuff the lamb shoulder, spreading the herb stuffing over the surface, leaving a 1-inch border. Season the herb mixture with salt and pepper. Roll the lamb up, tie it up with string at 1-inch intervals, brush it with olive oil and season the meat with salt and pepper. With a sharp knife, make small incisions in the meat and insert the halved garlic cloves.
Preheat the oven to 350 degrees F.
In a large oven-proof heavy skillet, heat the canola oil on high. Add the lamb shoulder to the skillet and sear on all faces until browned, about 10 minutes. Add the veal stock and garlic head. Season with salt and pepper. Remove from the heat and put in the oven for about 2 1/2 hours for medium, or using a meat thermometer until it reads 165 degrees F for medium or 170 degrees F for well done.
Remove the lamb shoulder from the skillet and keep warm. Mash the roasted garlic in the sauce. Strain over a medium-size stockpot and cook until reduced by half. Whisk in the butter. Season with salt and pepper and keep warm.
Slice the lamb shoulder and serve with the roasted garlic sauce and hash browns.