In a mixing bowl, combine olive oil, lemon juice, oregano, garlic powder, salt, and black pepper. Add the chicken breasts and coat well. Cover and marinate for at least 30 minutes.

Preheat the grill to medium-high heat. Grill the marinated chicken for about 6-7 minutes on each side or until fully cooked. Remove from heat and let rest for a few minutes before slicing.

In a bowl, mix Greek yogurt, grated cucumber, minced garlic, olive oil, lemon juice, dill, and salt until well combined. Refrigerate until ready to serve.

In each bowl, layer cooked quinoa, sliced grilled chicken, cherry tomatoes, cucumber, red onion, feta cheese, and olives. Drizzle with tzatziki sauce before serving.