Pat the lamb shoulder dry and rub with olive oil, garlic, rosemary, thyme, salt, pepper, and lemon zest.Place in a roasting pan and pour broth and lemon juice around the meat. Cover loosely with foil.Roast at 300°F (150°C) for 3.5–4 hours.Whisk honey, Dijon mustard, balsamic vinegar, and extra rosemary to make the glaze.Remove foil during the last hour and brush with half the glaze.Add remaining glaze after 30 minutes and raise heat to 375°F (190°C) for the final 15 minutes.Rest for 15 minutes and serve with pan drippings.