If you are using wooden skewers, soak them for at least 30 minutes.
Once you make the shawarma spice blend, set aside 1 tablespoon for the marinade and store the rest in a glass jar.
Trim the extra fat from the chicken thighs, cut each one into thirds, and put them into a large bowl. Combine with the grated onion, mayo, 1 1/2 tsp salt, and 1 tablespoon of the shawarma spice. Marinade the chicken for at least 4 hours.
Thread the chicken onto the skewers, trying not to squeeze the pieces too close together.
If you are using a charcoal grill, wait until the fire dies down a bit and the bricks turn white. Then place the chicken skewers on the grill, turning them every five minutes until internal temp reaches 165 F. It will take about 25-35 minutes.
Season the shashlik with a sprinkle of salt to taste and serve it with rice or pita bread, a fresh salad, and little bowls of zhoug sauce on the side. Enjoy!