Preheat oven to 180 degrees C. Coat bottom of a medium baking dish with about 1/4 cup olive oil.
In a small bowl, blend remaining olive oil, tomato paste, garlic powder, oregano, salt and pepper.
Arrange tilapia fillets in the prepared baking dish, skin side up. With a spatula, spread the olive oil and tomato paste mixture over the tilapia, heaping the majority in the centres of the fillets. Arrange onion and green capsicum slices on and around the fillets.
Bake in the preheated oven 15 minutes, until fish is easily flaked with a fork. Remove from heat, top each fillet with 1 tablespoon butter, and preheat the grill. Grill 5 to 7 minutes, until butter is melted and lightly brown.