In a large mixing bowl, whisk together all ingredients for the chicken marinade: 1/4 cup olive oil, 2 tbsp lemon juice, 1 tbsp white vinegar, 2 tsp ground cumin, 1.5 tsp smoked paprika, 1 tsp garlic powder, 1 tsp onion powder, 1/2 tsp ground turmeric, 1/2 tsp ground coriander, 1/4 tsp cayenne pepper (if using), 1 tsp salt, and 1/2 tsp black pepper.

Add the 1.5 lbs chicken thigh pieces to the marinade, tossing to ensure all pieces are well coated. Cover the bowl and refrigerate for at least 30 minutes, or up to 4 hours for deeper flavor.

Preheat your oven to 400°F (200°C). Line a large sheet pan with parchment paper for easier cleanup.

In a separate medium bowl, toss the 1 red onion wedges and 1 bell pepper strips with 1 tablespoon of olive oil, salt, and pepper to taste.

Spread the marinated chicken pieces evenly over one side of the prepared sheet pan. Arrange the seasoned onion and bell pepper on the other side. Ensure there is some space between ingredients for even roasting; avoid overcrowding.

Bake for 15 minutes.

Remove the sheet pan from the oven. Add the 1 cup cherry tomatoes to the pan and gently toss all the ingredients together. Return to the oven and bake for another 10-12 minutes, or until the chicken is cooked through (internal temperature 165°F/74°C) and the vegetables are tender-crisp and slightly caramelized.

While the chicken is roasting, whisk together the 1/4 cup tahini, 2 tbsp lemon juice, 1 minced garlic clove, and a pinch of salt in a small bowl. Gradually add 2-4 tbsp cold water, 1 tablespoon at a time, whisking continuously until the sauce reaches a smooth, pourable consistency. It should be thick but drizzle-able.

Remove the sheet pan from the oven. Let the shawarma rest for a few minutes. Serve immediately with warm pita bread, drizzled generously with the lemony tahini sauce, and garnished with fresh parsley. Offer optional sides like diced cucumber, hummus, or pickles.