In a medium bowl, whisk together Greek yogurt, minced garlic, lemon juice, olive oil, dried oregano, cumin, paprika, coriander, salt, and pepper to create the marinade.

Place chicken thighs into a large resealable bag or bowl. Pour the marinade over the chicken, ensuring thorough coverage. Seal or cover and refrigerate for at least 2 hours, preferably overnight.

Grate cucumber, squeeze out excess moisture, and mix with Greek yogurt, minced garlic, lemon juice, olive oil, dill, and salt and pepper to make the tzatziki. Refrigerate until serving.

Preheat grill to medium-high heat or oven to 400°F. Remove chicken from marinade, shake off excess, and grill for 6-7 minutes per side or bake for 25-30 minutes until cooked through.

Let the chicken rest for 5 minutes, then slice into strips.

Warm the pita on the grill for 1-2 minutes per side or in a skillet until pliable.

On each warm pita, spread tzatziki, layer with sliced chicken, and top with lettuce, tomatoes, cucumber, and onion. Optionally sprinkle with feta and serve immediately.