Marinate the Chicken: In a bowl, mix olive oil, lemon juice, minced garlic, oregano, salt, and pepper. Coat the chicken breasts and let them marinate for at least 15 minutes (or overnight for best results).
Cook the Chicken: Heat a grill pan over medium-high heat. Cook the chicken for about 5-6 minutes per side until fully cooked. Let it rest for a few minutes before slicing.
Prepare the Tzatziki Sauce: In a small bowl, combine Greek yogurt, grated cucumber, garlic, lemon juice, dill, salt, and pepper. Mix well and set aside.
Assemble the Bowl: Divide cooked quinoa or rice into four bowls. Top with sliced chicken, cherry tomatoes, cucumbers, red onion, olives, and feta cheese.
Drizzle with Tzatziki: Finish with a generous spoonful of tzatziki sauce and enjoy!