In a large mixing bowl, combine full-fat Greek yogurt, cumin, coriander, paprika, cinnamon, turmeric, cayenne, minced garlic, olive oil, lemon juice, and salt. Add the boneless, skinless chicken thighs, ensuring they are thoroughly coated with the marinade. Cover the bowl and let it marinate in the refrigerator for at least 30 minutes, or overnight for enhanced flavor.

Preheat your oven to 425°F (220°C) while you work on the vegetables. On a large sheet pan, arrange sliced red onions in a single layer, creating a flavorful bed for the chicken. Place the marinated chicken thighs on top of the onions, ensuring they are evenly spaced. Optionally, scatter halved cherry tomatoes around the chicken for added sweetness during roasting.

Slide the sheet pan into the preheated oven and roast for 25-30 minutes. You'll know the Chicken Shawarma is done when the thighs are golden brown and reach an internal temperature of 165°F (74°C). Check for tender, juicy chicken with a slightly crispy exterior.

While the chicken and vegetables are roasting, mix together chopped mini cucumbers, halved cherry tomatoes, and fresh parsley in a medium-sized bowl. Drizzle with extra-virgin olive oil, and season with a sprinkle of salt and pepper to taste.

In a small bowl, combine tahini with Greek yogurt, a bit more lemon juice, salt, and any additional spices you desire. Stir well until the mixture is creamy and smooth, adjusting consistency with water if necessary.

Once the chicken and vegetables are roasted and vibrant, remove the sheet pan from the oven. Serve the tender chicken and colorful roasted onions with the fresh cucumber salad and drizzle the tahini sauce on top. Add toasted pita on the side.