Mix all spices, salt, vegetable oil and the juice of 2 lemons in a large mixing bowl, add the chicken thighs and mix thoroughly until the chicken is evenly coated with the marinade. (Using your hands is the best way to do it.) Cover the bowl with plastic wrap and marinade overnight in the refrigerator.
Preheat the oven to 190C/375F skewer the chicken thighs on a shawarma spike and put in the oven for 30 minutes. After 30 minutes take out of the oven and wrap the chicken in tin foil to prevent the chicken from drying out, then return to the oven for 30 minutes longer or until the internal temperature reaches 165F/73C.
Remove from the oven and let it rest for 10 minutes, then shave slices of cooked chicken with a sharp knife to be served in a warm flatbread.
Notes
Alternatively, instead of individual spices listed you can use 6 tablespoons of the store bought shawarma spice mix.