In a mixing bowl, whisk together olive oil, lemon juice, minced garlic, oregano, paprika, salt, pepper, and red pepper flakes. Add chicken strips into the marinade, coating them evenly. Marinate for 15-45 minutes.

While the chicken marinates, chop cucumbers, tomatoes, and bell pepper. Combine with red onion, parsley, and crumbled feta cheese in a bowl. Season with salt and pepper, and let sit to meld flavors.

Heat a skillet over medium-high heat. Add marinated chicken and cook without moving for 3-5 minutes. Flip and cook for another 4-6 minutes until golden brown and cooked through.

Warm pitas using a hot skillet or oven until pliable.

To assemble, fill each warm pita with grilled chicken and vegetable salsa. Drizzle with tzatziki sauce and serve.