Heat a large pot over medium heat. Add 3 tablespoons of oil and the flour and stir into a paste. This is your roux.
Keep stirring for 10 to 12 minutes, until the roux turns light brown, about the colour of peanut butter. Don't stop stirring, or it can burn.
Add the bell pepper, onion and celery and cook for 5 minutes, stirring, until softened.
Add the tomatoes and garlic and cook for 1 minute more.
Stir in the stock, Creole seasoning, thyme, Worcestershire, hot sauce, salt and pepper. Bring to a boil, then lower the heat and simmer for at least 20 minutes so the flavours develop. It can simmer longer if you like.
While it simmers, heat 1 tablespoon oil in a pan over medium heat. Season the shrimp lightly with salt and pepper and cook for 2 to 3 minutes per side, until cooked through. (Or nestle the raw shrimp into the simmering sauce and cook for 5 minutes.)
Spoon the sauce over white rice or grits, top with the shrimp and garnish with parsley and chili flakes.