Preheat the oven to 325?F (160?C).

In a small bowl, mix together the cumin, coriander, cinnamon, allspice, black pepper, and salt.

Rub the spice mixture all over the lamb shoulder, ensuring it is well coated.

In a large roasting pan, heat the olive oil over medium heat. Add the sliced onions and cook until softened.

Add the minced garlic and cook for an additional minute.

Place the seasoned lamb shoulder on top of the onions in the roasting pan.

Pour the vegetable broth around the lamb and squeeze the lemon juice over the top.

Sprinkle the chopped parsley and mint over the lamb.

Cover the roasting pan with foil and place it in the preheated oven.

Roast for about 3 hours, or until the lamb is tender and easily pulls apart with a fork.

Remove the foil for the last 30 minutes of cooking to allow the lamb to brown.

Check the internal temperature of the lamb; it should reach at least 145?F (63?C) for medium-rare.

Once done, remove from the oven and let it rest for 15 minutes before carving.