Clean and pat the chicken thighs dry (substitute with chicken breast if you prefer but be carful not to over cook them). Place chicken in a ziplock bag or container with a seal.
Pour in lemon juice and olive oil. Mix until well coated.
In a small bowl: mix all the seasoning ingredients together. Pour the seasoning mixture into the ziplock bag or container with the chicken. Mix until evenly coated.
Let marinate for at least 3 hours, preferably overnight.
Tzatziki Sauce
Grate the cucumber and sprinkle ½ tsp salt on it. Let it sit for 10-15 minutes, this helps remove excess water. Squeeze the grated cucumber to remove additional water.
Place the drained cucumber, yogurt, minced garlic, lemon juice and olive oil in a medium sized bowl. Add a pinch/dash of salt and pepper to your liking. Mix well. Cover and refrigerate, until ready to use.
Grill
Preheat the grill or skillet to high.
Place the marinated meat on the gril and cook for 4-5 minutes on the first side (look for charring at the edges as thinner cuts cook faster). Flip and then cook for an additional 2-3 minutes.
Remove the meat from the grill and place in a container. Cover with aluminum foil and let stand/sit for ~10 minutes. When ready to eat, slice the chicken. (see video and pics for reference)
Layering the Shawarma
Preheat the oven to 275°F (135°C).
Place the pita bread in the oven for 1-2 minutes or until warm (careful not to over heat, you don't want the bread to become "toasted"). Remove the pita bread from the oven.
Fill the pita bread in this order: Nice thick layer of Tzatziki Sauce, chopped lettuce, sliced tomatoes and chicken meat. (see video for reference)