Preheat the oven to 400°F (200°C).
In a small bowl, whisk together the olive oil, lemon juice, oregano, garlic powder, paprika, salt, and pepper.
Place the fish fillets in a baking dish and pour the olive oil mixture over them, ensuring they are well coated.
Scatter the cherry tomatoes, Kalamata olives, and red onion around the fish in the baking dish.
Lay lemon slices on top of the fish and vegetables.
Bake in the preheated oven for 20-25 minutes, or until the fish flakes easily with a fork and is cooked through.
Remove from the oven and let it rest for a few minutes before serving. Garnish with fresh parsley.