In a bowl combine the Greek Yogurt with za’atar, sumac, garlic, chili powder, and a good pinch of salt and pepper.
Add the chicken thighs and give it a good mix. Cover with plastic wrap and Refrigerate for at least 2 hours or if you can up to 24 hours.
In a small bowl combine the tahini paste, Greek yogurt, and lemon juice. Give the mixture a good stir, if it is too thick then add a bit more yogurt to thin it out. Mix in the sumac, chili powder, and salt to taste. keep aside
Thread the marinated chicken on to the wooden skewers and set aside on a plate. Heat up a stove top grill pan and brush it with some vegetable oil. Grill the chicken for 10 minutes, make sure to flip the chicken (if you find that the chicken is sticking it usually means it’s not done yet and let it cook for a few more minutes till it releases naturally).
Let the chicken rest for a few minutes, squeeze some lemon on top if you like and serve with spicy tahini sauce.