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 except for the red onions. 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.
Oven
Preheat the oven to 425°F.
Add sliced red onions to the marinated chicken. Mix everything together.
Grease the baking sheet (13" x 18") with olive oil. Place chicken in a single layer on the baking sheet. Place the onions around the chicken (not on top of the chicken).
Cook for 35 minutes or until the internal temperature reaches to 165°F . If you want the chicken to be a little more crispy, broil on high for 2-4 minutes (keep an eye on it). Rest for 5 minutes before slicing.
Tzatziki Sauce
Grate the cucumber and sprinkle ½ tsp salt on it. Let it sit for 10 minutes, this helps remove excess water. Squeeze the grated cucumber to remove additional water.
Place the drained cucumber, yogurt, minced garlic, lemon juice, dill and olive oil in a medium sized bowl. Add a pinch/dash of salt and pepper to your liking. Mix well.
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, sliced cucumbers and top with chicken. (see video for reference)