Melt the butter in a large heavy-based pot over MEDIUM heat. Sprinkle in the flour. Use a rubber spatula to stir for 12–15 minutes, until the butter-and-flour mix turns caramel brown.
Add the celery, green pepper, and onion to the pot with the butter-and-flour mix, stir. Cook for 5–8 minutes.
Add the sausage, garlic, tomato, chicken stock, bay leaves, salt, and pepper. Stir well. Bring to a boil, then turn down the heat and let simmer for an hour.
Add the shrimp. Cook for another 5 minutes, until the shrimp turns pink.
Sprinkle the green onions over and serve with rice.