1
Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium baking dish with cooking spray.
2
In a small bowl, blend olive oil, tomato paste, garlic, oregano, salt and pepper.
3
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 centers of the fillets. Arrange onion and green pepper slices on and around the fillets.
4
Bake in the preheated oven 10 minutes, until fish is easily flaked with a fork. Remove from heat, top each fillet with 1/2 tablespoon butter, and set oven to broil. Broil 5 minutes, until butter is melted and lightly brown.