In a large bowl, whisk together the olive oil, juice from half the lemon, minced garlic, paprika, cumin, coriander, turmeric, cinnamon (if using), salt, and black pepper to make the spice marinade.

Pat the chicken pieces completely dry with paper towels (this helps get that crispy skin). Add the chicken to the marinade and toss until every piece is well coated. If you have the time, let it marinate for at least 30 minutes.

Preheat the oven to 400°F (200°C). Arrange the marinated chicken on a large rimmed baking sheet, skin-side up, spacing the pieces out so they roast instead of steam. Bake for 25 minutes.

While the chicken cooks, add the rinsed chickpeas and red onion wedges to the empty marinade bowl. Toss to coat in any leftover marinade, adding an extra drizzle of olive oil if needed.

Carefully remove the hot pan from the oven and arrange the chickpeas and onion around the chicken. Return the sheet pan to the oven and roast for another 20–25 minutes.

The shawarma is ready when the chicken is cooked through and the skin is golden and crisp. Let everything rest for about 5 minutes, then squeeze the remaining lemon juice over the whole pan just before serving.