Place the oil in a pot set over medium heat. Add the onion, green pepper and celery and cook 3 to 4 minutes to soften. Add the stewed tomatoes, wine, orange juice, stock, tomato paste and potatoes and bring to a simmer. Simmer the stew for 10 minutes, or until the potatoes are cooked. Stir in the fish and cook 4 minutes more, or until just cooked through. Stir in the pesto, salt and pepper and serve.