In a 5 quart crock pot, place the chicken breasts onto the bottom of the pot. Zest and juice the lemon over the chicken. Sprinkle the seasonings over the chicken and cook for 3-5 hours on medium heat, or until chicken is tender. When chicken is cooked, shred the chicken into large pieces and keep the chicken in the juices. Season with more salt and pepper as desired.
Slice the tomatoes, lettuce and red onions into thin strips. Prepare the tzyaki sauce by combining all of the ingredients together in a bowl. Serve the sauce over the shredded chicken, tomatoes, onions and cucumbers in a tortilla or wrap.