In a mixing bowl, combine olive oil, minced garlic, dried oregano, ground cumin, salt, and pepper. Add the boneless, skinless chicken breasts, ensuring they are thoroughly coated in the marinade. Cover the bowl with plastic wrap and let the chicken marinate in the refrigerator for at least 30 minutes.

Preheat your grill or non-stick skillet to medium-high heat. Once hot, place the marinated chicken breasts onto the grill and cook for about 6-7 minutes per side, or until the internal temperature reaches 165°F (75°C) and the chicken is no longer pink inside. After grilling, let the chicken rest for 5 minutes before slicing it thinly.

While the chicken is grilling, take a small bowl and whisk together plain Greek yogurt and fresh lemon juice until smooth. Adjust seasoning as needed.

In another mixing bowl, combine diced tomatoes, diced cucumber, red onion, Kalamata olives, and crumbled feta cheese. Gently toss until evenly mixed.

In a dry skillet over medium heat, warm the corn tortillas or soft taco shells for about 30 seconds on each side, just until they become pliable and slightly toasted.

Place a few sliced pieces of the grilled chicken on each warm tortilla. Top generously with the fresh salad mixture, then drizzle the yogurt sauce over everything.

Finish with a sprinkle of chopped parsley for a fresh touch, and serve immediately.