Put a large pot over medium high heat and add the butter.
Once the butter has melted and is bubbling, add the onions and cook for 1-2 minutes, stirring often.
Add the celery, leeks, and garlic, and cook for another 1-2 minutes.
Turn the heat down to medium low and add the flour and cook for 3-4 minutes, stirring constantly.
Add the potato, thyme, salt, fish stock, and cream, turn the heat up to high and bring everything to a boil. Once it has boiled, turn the heat down to low and simmer for 15-20 minutes, until the potatoes have softened.
Finally add the cubed codfish and cook for another 4-5 minutes and serve.