Preheat your oven to 300°F (150°C).
In a small bowl, mix minced garlic, chopped rosemary, thyme, olive oil, salt, and pepper to form a paste.
Rub the herb mixture all over the lamb shoulder, making sure to coat all sides evenly.
Place the lamb in a roasting pan and surround it with quartered onions.
Pour the broth around the lamb, making sure not to wash off the herb mixture.
Cover the roasting pan tightly with aluminum foil.
Roast in the preheated oven for 4 to 5 hours, or until the meat is tender and pulls away from the bone.
Remove the foil for the last 30 minutes of cooking to allow the lamb to develop a golden crust.
Once done, let the lamb rest for at least 20 minutes before carving.
Slice the lamb shoulder and serve with the roasted onions and pan juices.