Add the chicken to a large bowl and set aside.
Mix the yogurt, olive oil, lemon juice, red wine vinegar, garlic paste, oregano, paprika powder, salt and pepper together in a small bowl and pour it over the chicken and stir to combine.
Cover and marinate in the fridge for 30 minutes or up to 8 hours.
Heat a large skillet over medium heat and add in the chicken strained from the marinade.
Cook on each side for about 6 minutes or until light and golden on each side.
Remove the chicken from the pan and let it rest for 5 minutes then slice into long strips and set aside.
While the chicken is cooking, toast the pitas in a skillet over medium heat for one minute on each side.
Lay the pitas out and layer on the romaine lettuce, sliced tomatoes, cucumber, onion and sliced chicken.
In a small bowl, mix together the tzatziki sauce ingredients until smooth, drizzle it on the chicken and serve immediately.