In a medium bowl, whisk together the olive oil, dried oregano, garlic powder, onion powder, smoked paprika, sea salt, and freshly cracked black pepper to create a flavorful marinade. Add the chicken breasts to the marinade, ensuring they are thoroughly coated on all sides. Cover the bowl with plastic wrap and let it marinate in the refrigerator for at least 30 minutes, or preferably for 1-2 hours to enhance the flavors.
Heat a grill pan or a skillet over medium-high heat until hot. Carefully place the marinated chicken breasts onto the heated pan. Cook for approximately 6-7 minutes on each side, or until the chicken is fully cooked and no longer pink in the center (an internal temperature of 165°F/75°C). Once cooked, remove the chicken from the heat and allow it to rest for about 5 minutes before slicing into strips.
While the chicken is resting, prepare the serving bowls. Begin by adding a generous base of cooked quinoa or fluffy rice to the bottom of each bowl.
Add a vibrant layer of chopped romaine lettuce on top of the grains. Then, distribute diced tomatoes, diced cucumber, and sliced red onion evenly among the bowls, creating a colorful and fresh presentation.
Once the chicken has rested, slice it into thin strips. Arrange the sliced chicken on top of the vegetable layers in each bowl, making it a star of the dish.
Sprinkle crumbled feta cheese over each bowl liberally, and top with a generous dollop of creamy tzatziki sauce for a refreshing finish.
To add a pop of color and freshness, finish each bowl with a sprinkle of chopped fresh parsley as a garnish.