Preheat the oven to 375°F (190°C) and lightly grease a 9x13 inch ceramic or glass baking dish.
In the baking dish, combine the uncooked white rice, chicken broth, minced garlic, lemon juice, dried oregano, salt, and black pepper.
Distribute the cubed chicken breast evenly throughout the rice and broth mixture.
Cover the dish tightly with a layer of heavy-duty aluminum foil to ensure steam is trapped for proper rice hydration.
Bake for 40 to 45 minutes, or until the liquid is fully absorbed and the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven and allow the casserole to rest, covered, for 5 minutes.
In a separate small bowl, whisk the Greek yogurt with the fresh chopped dill.
Uncover the casserole and fluff the rice with a fork, then fold in the yogurt and dill mixture until creamy.
Top the casserole with the diced cucumber, crumbled feta, sliced olives, and red onion before serving.