In a large mixing bowl, combine the ground cumin, coriander, cinnamon, paprika, turmeric, allspice, salt, and pepper. Add the garlic-infused oil to the spices and mix to form a thick paste.

Thinly slice the lamb shoulder into strips. Add the lamb to the spice mixture and toss well to coat every piece evenly. Make sure each slice is thoroughly covered in the marinade. Cover the bowl with cling film and let the lamb marinate for at least 20 minutes at room temperature, or refrigerate it overnight for maximum flavor. If refrigerating, allow the lamb to come to room temperature before cooking.

Preheat the oven to 200°C (390°F). Line a large baking tray with parchment paper or lightly grease it to prevent the lamb from sticking.

Spread the marinated lamb slices in a single layer on the prepared baking tray. Ensure there is enough space between the slices for even cooking. Place the tray in the preheated oven and roast for 20 minutes. Flip the lamb slices halfway through the cooking time to ensure they cook evenly. Continue roasting for another 15-20 minutes, or until the lamb is fully cooked and crispy at the edges.

Once cooked, remove the lamb from the oven and let it rest for a few minutes to lock in the juices. Serve the lamb hot, alongside plain yogurt (optional) for added creaminess. Garnish with freshly chopped parsley for a burst of freshness.

Serve the lamb shawarma with warm pita bread, fluffy rice, or over a fresh salad. You can also use it as a filling for shawarma wraps with vegetables and sauces like tahini or tzatziki.