In a large zip-top bag, combine the lemon juice, 1/2 cup olive oil, garlic, salt, pepper, cumin, paprika, oregano and red-pepper flakes. Add the chicken, zip the top of the bag and give it a quick shake to combine. Refrigerator for at least 1 hour and up to 12 hours.
Preheat a cast iron to medium high heat. Remove the chicken from the marinade and grill each chicken thigh for 4-5 minutes on each side until done. Remove and set aside to rest.
Slice the chicken against the grain into thin strips and transfer back into the skillet and sauté for 2 minutes until just slightly crispy around the edges.
Transfer the chicken to a large bowl and toss to combine with the parsley, cherry tomatoes and cucumbers.
Divide the shredded lettuce amongst 4 bowls and top with a hefty portion of the chicken tomato mixture. Drizzle wtih tzatziki or serve it on the side. Also pita chips on the side.