Marinate the chicken: In a large bowl, whisk together garlic, cumin, coriander, paprika, turmeric, black pepper, salt, cardamom, cloves, olive oil, and lemon juice. Add the chicken thighs and turn to coat evenly. Marinate for at least 30 minutes or up to overnight.
Preheat the oven: Set oven to 425°F.
Add vegetables: Add cauliflower, bell peppers, and red onions to the same bowl and toss until coated in the remaining marinade.
Roast: Spread the chicken and vegetables in a single layer on a large sheet pan. Roast for 30–40 minutes, until the chicken reaches an internal temperature of 165°F and the vegetables have started to take on some color.
Make the yogurt sauce: While the chicken cooks, combine yogurt, lemon juice, olive oil, salt, and garlic in a small bowl. Stir until smooth and creamy.
Serve: Plate the chicken and roasted vegetables and drizzle with the yogurt sauce. Alternatively, stuff everything into a warm pita with lettuce and tomato for a handheld shawarma wrap.