Preheat the oven to 400°F (200°C) and lightly brush a medium baking dish with a little olive oil to prevent sticking.
Place the sliced chicken breasts in a mixing bowl and season with the remaining olive oil, garlic powder, dried oregano, salt, and black pepper, rubbing the seasoning evenly over each piece.
Arrange the seasoned chicken in a single layer in the prepared baking dish.
Scatter the chopped onion over the chicken, then layer the sliced zucchini and diced tomatoes on top and around the sides, tucking some pieces between the chicken so each serving has vegetables.
Sprinkle the mozzarella cheese, Parmesan cheese, and crumbled feta evenly over the vegetables and chicken.
Bake the Mediterranean Chicken Zucchini Bake in the preheated oven for 25–30 minutes, or until the chicken is cooked through to 165°F and the cheese is melted and lightly golden.
If you like a crispier top, switch the oven to broil for 2–3 minutes at the end, watching closely so the cheese does not burn.
Remove the baking dish from the oven and let the Mediterranean Chicken Zucchini Bake rest for 5–10 minutes to allow the juices to settle.
Garnish with chopped fresh herbs and serve with lemon wedges, grains, or a simple green salad for a well balanced dinner.