Heat oil and make a dark roux with flour. Combine onion, celery, green pepper and garlic to make a mirepoix and add to roux. Cook until vegetables are tender, about 10 minutes. Stir in tomatoes and their juice, shrimp stock and bay leaves. Bring to a boil, reduce heat and simmer 1 hour. Add shrimp and cook until pink. Season to taste with salt and cayenne. To serve, ladle over steamed rice and garnish with parsley.