01 - Whisk together olive oil, lemon juice, minced garlic, dried oregano, salt, and black pepper in a bowl. Add chicken breasts, turning to coat evenly. Let marinate for 15 minutes to 1 hour for optimal flavor absorption.
02 - Combine Greek yogurt, olive oil, lemon juice, grated and drained cucumber, minced garlic, chopped fresh dill, salt, and pepper in a small bowl. Mix thoroughly until smooth and well incorporated. Refrigerate until ready to serve.
03 - Preheat grill pan or skillet over medium-high heat. Cook marinated chicken for 5-6 minutes per side until internal temperature reaches 165°F and juices run clear. Transfer to cutting board, let rest for 5 minutes, then slice into thin strips.
04 - Heat flatbreads in a dry skillet or preheated oven for 1-2 minutes until warm and pliable. Avoid over-toasting to maintain flexibility for folding.
05 - Spread generous layer of tzatziki sauce over each warmed flatbread. Layer baby spinach or arugula, sliced grilled chicken, cherry tomato halves, cucumber slices, red onion rings, Kalamata olives, and crumbled feta cheese evenly across surface.
06 - Drizzle additional tzatziki sauce over toppings if desired. Serve immediately while flatbreads are warm and chicken is at optimal temperature.