In a large bowl, mix together all of the Lemon Chicken ingredients except for the chicken. Stir until evenly combined. Toss the chicken in the mixture until well-coated and set aside.
Meanwhile, combine all of the Tzatziki ingredients and set aside.
Heat a cast iron grill pan or skillet over medium-high heat. Once hot, cook the chicken (in batches if necessary), until cooked through, about 3 minutes per side.
Serve the chicken on warmed pita with lettuce, tomato, onion, and cucumber slices, and plenty of tzatziki, to top.