Make a roux: Combine flour and shortening or oil in a large pot on medium to high heat. Cook, stirring frequently, until flour is deeply browned, but not burned. Compare to color of Hershey's chocolate bar.
Add garlic and cook until golden brown.
Add onion, bell pepper, and celery and cook until transparent.
Add shrimp and tomato sauce, simmer 10 minutes, stir in water, and mix well.
Add all other ingredients except okra and oysters, and cook for about an hour.
Add okra, and cook 20 minutes.
Add oysters and cook 10 minutes longer.
Serve in a soup bowl over steamed rice.
Add Crystal Hot Sauce to taste