In a mixing bowl, whisk together olive oil, lemon juice, minced garlic, oregano, paprika, salt, pepper, and red pepper flakes. Cut chicken into bite-sized pieces and add to marinade, coating well. Marinate for 15-45 minutes.

Combine diced cucumbers, tomatoes, bell pepper, red onion, parsley, and feta cheese in a large bowl. Drizzle with lemon juice, salt, and pepper. Let sit for about 10 minutes.

Heat a skillet over medium-high heat. Cook marinated chicken in a single layer for 3-5 minutes, then flip and cook for another 3-4 minutes until cooked through, reaching an internal temperature of 165°F.

Warm pitas in a dry skillet, over low flame, or wrapped in foil in a preheated oven at 350°F for about 5-10 minutes.

Fill each warm pita with cooked chicken, top with fresh salsa, and drizzle with tzatziki sauce. Enjoy!