In a bowl, combine the minced garlic, lemon juice, olive oil, dried oregano, dried thyme, ground cinnamon, ground cumin, ground allspice, salt, and pepper.
Place the lamb in a large dish or a ziplock bag.
Pour the marinade over the lamb, ensuring it is well coated. Cover and refrigerate for at least 2 hours, preferably overnight.
Preheat your oven to 160째C (320째F).
In a large roasting pan, place the sliced onion and quartered potatoes.
Drizzle with olive oil and season with salt and pepper. Toss to coat.
Place the marinated lamb on top of the vegetables.
Pour the white wine over the lamb and vegetables.
Add fresh rosemary sprigs, if using.
Cover the lamb and vegetables with a large sheet of parchment paper.
Securely wrap the parchment paper with aluminum foil, sealing all the edges to create a tight package.
Place the roasting pan in the preheated oven and bake for 3 to 3.5 hours, or until the lamb is tender and falling off the bone.
Carefully remove the foil and parchment paper from the roasting pan.
Increase the oven temperature to 200째C (390째F) and return the uncovered lamb to the oven for an additional 20-30 minutes to brown the meat.
Enjoy your meal!