Place halibut fillets on a large sheet of aluminum foil and season with Greek seasoning. Combine tomato, onion, olives, capers, olive oil, lemon juice, salt, and pepper in a bowl. Spoon tomato mixture over the halibut. Carefully seal all the edges of the foil to create a large packet. Place the packet on a baking sheet.
Bake in the preheated oven until the fish flakes easily with a fork, 30 to 40 minutes.