Preheat oven to 375°F (190°C).
Sauté diced onion in olive oil or butter until softened (skip the garlic here since we’re using powder).
In a large bowl, combine chicken, frozen vegetables, cooked rice, garlic powder, sautéed onion, oregano, thyme, lemon juice, salt, and pepper.
Mix everything well and transfer to a greased baking dish.
Top with crumbled feta or mozzarella if desired.
Bake for 25–30 minutes, until hot throughout and lightly golden on top.