Bring to stock to a simmer in a saucepan. Season the minced fish with salt and pepper and form into small balls, about the size of a walnut.
Add the fish balls and potatoes to the simmering stock.
Meanwhile, saute the onion and garlic in butter until the onion is transparent. Add to the simmering soup.
Mix the milk and flour together until smooth. Pour in a thin stream into the soup, stirring constantly. Season with salt and pepper to taste and simmer until the potatoes are done.