In a gallon ziploc bag, combine the olive oil, lemon juice, garlic, Shawarma Spice and salt. Add the chicken thighs and massage the outside of the bag to move them around until they are well coated with marinade. Refrigerate and allow to marinate 1-2 hours or overnight.
Preheat the grill to 375 degrees F and cook with the lid down, turning every 10 minutes or so, until the internal temperature reaches 165 degrees, about 30 minutes total.
Allow the chicken to cool slightly and chop into bite sized pieces. Combine the chicken, and juices from the board, with the tzatziki, onion, tomatoes and cucumber in a large bowl.
Serve atop chopped romaine, with a garnish of herbs.