1Melt butter in a 5-quart saucepan over medium heat. Cook and stir onion, celery, and garlic in melted butter until onion is tender, 5 to 7 minutes.
2Stir flour into the vegetable mixture until the vegetables are evenly coated. Pour clam juice and water into the saucepan; whisk until smooth. Add potatoes, bay leaf, and thyme to the mixture. Bring the liquid to a boil, reduce heat to medium-low, and cook at a simmer until the potatoes are tender, about 15 minutes.
3Gently stir cod into the soup. Place a cover on the saucepan and cook until the cod is flaky, about 8 minutes.
4Pour milk into the soup; stir. Simmer soup until hot, 1 to 2 minutes. Season with salt and black pepper. Remove bay leaf to serve.
Footnotes
Tip
Aluminum foil helps keep food moist, ensures it cooks evenly, keeps leftovers fresh, and makes clean-up easy.