In a large bowl, add the ingredients for the marinade (yogurt, lemon juice, olive oil, garlic, tomato paste, all the seasoning and salt/pepper) and whisk to combine.
Place the chicken cubes in the same bowl and coat the chicken in the marinade. Cover, and let rest/marinate in the fridge for at least 4 hours.
Thread the chicken on skewers right before grilling. Add one wedge of onions to each skewer, if desired.
Grill on medium heat for 5-8 minutes per side.
Serve with pita bread, lettuce and garlic sauce, if desired.