In a medium-sized bowl, add the za'atar seasoning, paprika, olive oil, lemon juice, lemon zest, minced garlic, salt, and black pepper, and mix well. Add the chopped chicken breasts and toss to coat. Cover and place in the fridge for a minimum of 30 minutes and up to 12 hours.
Preheat the oven to 350 degrees. Place a cast-iron or large skillet on the stovetop over medium heat. Add a drizzle of cooking oil to the skillet. Once hot, remove the pieces of chicken from the marinade and sear on both sides. Keep the remaining marinade. Don't throw it away! Do not worry about cooking the chicken all the way. It will finish in the oven.
Once the chicken is browned on both sides, move to the edge of the pan and add the yellow onion. Saute for about one minute. Add the remaining marinade to the pan along with the lemon juice, chicken stock or broth, and the grain-free orzo. Move the chicken around in the liquid so no orzo is on top and that the pasta is covered. Place the entire skillet in the oven and set a timer for 15 minutes.
While the chicken is cooking, make the herbed tahini by adding all the ingredients to a food processor or food blender and blend until smooth and creamy. Set aside until the chicken is cooked.
Once the timer goes off and the chicken has been in the oven for 15 minutes, remove and allow to cool slightly before serving. Drizzle with the herbed tahini and enjoy!