Season the chicken thighs with the za'atar, cumin, garlic powder, salt and pepper.
Heat a large braiser pan or skillet on medium heat with a generous amount of olive oil. Sear the chicken skin side down for about 4 minutes to brown, then remove the chicken from the pan.
Add in the diced yellow onion and garlic to the remaining oil and saute for 5-7 minutes until soft.
Add in the pearled couscous, mustard and spices and toss together.
Pour the broth or water into the pan, then nestle the chicken pieces into the couscous.
Cover the pan and bring it to a boil, then reduce the heat to low and simmer for about 35 minutes, until mostly all of the liquid is absorbed. Set the pan aside to rest for 10 minutes before fluffing the couscous with a fork.
While the chicken and couscous cook, prepare the olive salsa by adding all of the ingredients to a small food processor and pulsing together until finely chopped. Alternatively, you can finely dice the olives, herbs and shallots and mix with the remaining ingredients in a bowl to create the salsa instead of blending.
Spoon the olive salsa over the dish when serving with fresh chopped parsley and a squeeze of lemon juice.