I have eaten this soup for many years. The recipe was given to me by my late mother-in-law who was of Scandinavian decent. Her son's used to fish for brooke trout and she would use that in her soup. I can well imagine the Finns used to use goat butter and milk to make this soup in the old country as I did here. This is definitely one of my favourite soups!
Clean the potatoes and cut in half. Cook for 7-10 minutes in a large stew pot. Make sure you check them. Stick a fork in them at the 7 minute mark, if they are still a bit hard, that's fine, take them off the heat and remove from the water. Keep the cooking water!!!
Put the fish and bay leaves into the potato water and cook for 10-15 minutes. Then, remove the bay leaves when done and set the fish aside.
Pour the potato/fish water into another pot.
Using the pot you just emptied, add the butter and saute the scallions, celery, thyme and salt and pepper. Don't let it brown, just cook until the scallions are soft. Add the flour and stir gently.
Add the water back into the pot with the butter, scallions, celery and thyme and add the milk. Simmer this on medium for about 10-15 more minutes. This will thicken slightly.
While this is simmering flake the fish and add it with the potatoes back into the liquid.