Step 1: Slice the chicken thighs into thin, even strips, about 1/4 to 1/2 inch thick.
Step 2: In a large bowl, whisk together the yogurt, olive oil, lemon juice, minced garlic, cumin, coriander, turmeric, paprika, onion powder, salt, black pepper, and cayenne pepper (if using) to create the marinade.
Step 3: Add the sliced chicken to the marinade, ensuring all pieces are well coated. Cover the bowl and refrigerate for at least 4 hours, or preferably overnight, for best flavor.
Step 4: Preheat your oven to 425°F (220°C). Line a large baking sheet with parchment paper or foil. If you have a wire rack, place it on the baking sheet.
Step 5: Spread the marinated chicken in a single layer on the prepared baking sheet or wire rack. Do not overcrowd; cook in batches if necessary.
Step 6: Bake for 20-25 minutes, flipping the chicken halfway through, until cooked through, golden brown, and slightly charred at the edges. For extra crispiness, you can broil for 2-3 minutes at the end, watching carefully.
Step 7: Remove from oven and let the chicken rest for a few minutes before serving. This helps retain the juices.
Step 8: Serve the chicken shawarma in warm pita bread with lettuce, tomatoes, cucumbers, red onion, pickled turnips, hummus, and a generous drizzle of garlic sauce. Garnish with fresh parsley and a sprinkle of sumac.