Heat the oil in a large skillet over medium heat. Add the onion, bell pepper and garlic and cook until tender, about 5 minutes. Add the remaining ingredients, except cod and green onion, bring to a simmer, and simmer 10 minutes. Nestle the pieces of cod into the sauce, cover and cook 6 to 8 minutes, or until cooked through. Sprinkle with green onion and serve.