Add chicken to a large bowl or ziplock bag. Combine marinade ingredients and pour over chicken. Stir well to coat and marinate for 1-4 hours.
Meanwhile, make the tzatziki sauce by combining all ingredients. Taste it and add more seasoning (lemon juice, garlic, salt, pepper or cumin)if needed. Refrigerate.
Heat grill to medium and oil the grates (or if cooking on the stove, add a little oil). Remove chicken from marinade, letting any excess sauce drip off.
Grill chicken for a few minutes on each side, flipping once, until cooked through. Remove to a plate to rest before chopping into bite-size pieces.
Toast or warm your pitas. Layer with chicken, lettuce, tomato, red onion, cucumber and sauce. Enjoy!
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Notes
*I like to use chicken breast filets, or if you're chicken breasts are really thick you can slice the breast in half, lengthwise (called "butterfly") to make two thinner pieces. Make ahead instructions: The marinade for the chicken and tzatziki sauce can be made 2-3 days in advance. Freezing Instructions: The raw chicken can be frozen in the marinade for up to 3 months (preferred method). Thaw overnight in the fridge, then cook chicken according to instructions. The cooked chicken can also be frozen for up to 3 months. Thaw overnight in the fridge, then rewarm.