Heat butter in a large stockpot over medium. Add onion, carrots, celery, salt and pepper and cook, stirring occasionally, 5 minutes.Advertisement
Stir in garlic and cook 2 minutes. Sprinkle with flour and cook 1 minute.
Whisk in stock and milk. Stir in potatoes and thyme sprigs. Increase heat to medium-high and bring to a boil. Reduce to medium-low and simmer 10 minutes. Stir in cod, cover and cook 5 minutes, until fish is opaque. Remove thyme stems and serve.