Make the tzatziki sauce. See above for a link to the recipe.
Whisk together the olive oil, lemon juice, red wine vinegar, minced garlic, oregano, and a pinch of crushed red pepper, to taste in a Ziploc bag. Add the chicken breasts and marinate in the refrigerator for 1 hour. Remove from the ziplock bag and place on a plate. Season both sides with sea salt and freshly cracked pepper, to taste.
Preheat a grill pan that has been coated in cooking spray over medium heat. Place the seasoned chicken in the grill pan and cook for 8-10 minutes before flipping over and cooking for an additional 7-8 minutes, or until the chicken has cooked through.
Allow the chicken to rest for a few minutes on a cutting board before slicing into thin strips.
Warm your pita bread in the microwave for 20 seconds.
Top the pita with the tzatziki sauce, sliced chicken, mixed greens, tomatoes, red onion, and feta cheese. Wrap in tin foil and serve. Enjoy.