Cut 1 1/2 pounds halibut steaks into single servings. Sprinkle with salt and pepper. Cut six green pepper rings and place on top of each serving. In each ring place 1/2 teaspoon chopped onion. Combine 1/2 cup ketchup with 4 tablespoons butter. Heat until butter is melted. pour carefully over fish steaks. Bake at 425 degrees for 30 minutes.