In a gallon-size bag, combine 1-2 lbs chicken thighs (bone-in or boneless), ½ cup Greek yogurt, 2 tbsp olive oil, 1 tbsp minced garlic, juice and zest of 1 lemon, 1 tbsp Greek seasoning, and 2 tsp dried oregano. Seal the bag and refrigerate for at least 30 minutes.
Preheat a griddle or grill pan. Add the marinated chicken thighs, cooking them on low heat if bone-in (for even cooking). Flip the chicken after 3-4 minutes, using a lid or dome to help with convection cooking. Continue cooking for 10-15 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
While the chicken cooks, prepare the salad. In a large bowl, combine 1 English cucumber (chopped into half moons), ½ cup grape tomatoes (sliced in half), ¼ red onion (thinly sliced), 1 tbsp Meyer lemon vinegar (or red wine vinegar), 1 tsp salt, ½ tsp pepper, juice of 1 lemon, 2 tsp dried oregano, and 1 tbsp olive oil. Toss well and set aside.
Once the chicken is cooked, remove it from the pan and set aside. In the same pan, use the chicken fat to toast 4-5 pitas for 1-2 minutes on each side, until golden and crispy.
To build each pita, spread a generous amount of tzatziki on the pita, then add the cooked chicken, cucumber salad, and a sprinkle of feta cheese (optional).