Melt the butter in a heavy-based pot over medium heat. Sprinkle in the flour. Using a rubber spatula, mix the roux (i.e., the flour and butter mix) until it turns into a deep caramel color.
Toss in the prepared onion, green pepper, and celery and stir, cooking for another 5 minutes.
Add the garlic, Creole spice, tomatoes, stock, bay leaves, and a pinch of salt and pepper and stir. Bring the mix to a boil, then turn down the heat, and let the sauce simmer for 45 minutes. Stir occasionally to prevent it from sticking to the bottom of the pot.
Add the shrimp and stir. Cook the shrimp for 5 minutes or until they turn pink. Take out the bay leaves and stir in the green onions.