- In a large pot over medium heat, soften the onion in the oil. Add the celery, bell pepper and garlic and cook for 5 minutes. Add the toasted flour and cook for 1 minute, stirring to coat the vegetables. Add the broth while whisking to emulsify the soup. Add the tomato and Old Bay, and season with salt and pepper. Let simmer for 30 minutes.
- Reduce the heat and add the shrimp. Let simmer for 6 to 8 minutes or until fully cooked. Adjust the seasoning. Ladle the soup into bowls and top with the green onion. Add cooked rice to the soup, if desired.
To replace Old Bay seasoning, grind 1 tbsp black peppercorns, 1 tbsp celery seeds and 6 bay leaves in a spice mill. Add 1 tsp cayenne pepper. The spice mix will keep for 3 months in an airtight container at room temperature.