In a 2-gallon heavy saucepan, heat oil over medium-high heat. Whisk in flour, stirring constantly, until a light brown roux is achieved. Add onions, celery, bell peppers, and garlic. Sauté 3 to 5 minutes until vegetables are wilted and onions transparent. Blend in the tomato sauce and diced tomatoes. Slowly add in the shellfish stock, stirring constantly until a sauce-like consistancy is achieved. Cook approximately 15 minutes, stirring occasionally. More stock may be added if mixture becomes too thick. Add shrimp, green onions, and parsley and cook 5 more minutes. 

Season with salt and pepper and serve over steamed white rice with hot sauce on the side.