Think of this recipe as a guide. You can use pretty much any fish you want, even non-white fish like trout and salmon, and you can change up the broth to suit your taste. A clear fish broth would be lovely, but even regular chicken broth would be good. Of course, this mushroom broth is to-die-for, so if you can assemble the ingredients, by all means. But don't let the broth scare you off. It's the dumplings that are the key here.
Start with the broth, if you are making it. Saute the carrot, celery and onion in the olive oil until they are soft and the onion is translucent, about 5 to 7 minutes. Add the black garlic (or roasted garlic) cloves and mash them into the vegetables.
Add the mushrooms and pour over the broth and the water. Bring to a simmer, cover the pot and simmer gently for at least 90 minutes, and 2 hours is better. Strain the broth through a paper towel set in a fine-meshed strainer into a bowl. Season it with the soy sauce. Pour into a clean pot and keep warm.
To make the dumplings, soak the crustless bread in the milk for a bit, and then mash it into a paste with a fork. Run the fish through the fine die of a meat grinder, of chop it coarsely and buzz it into bits with a food processor; don't let it become a paste, though.
Mix the fish into the bread paste with the remaining dumpling ingredients. Bring a large pot of salted water to a boil. Form quenelles or little balls of the pike mixture -- use teaspoons for soups like this, tablespoons for serving by themselves -- and drop them into the boiling water. When the dumplings float, cook them for a minute more and then put them in the bottom of soup bowls.
When all the dumplings are made and in the bowls, bring the broth to a bare simmer and pour over the dumplings in the bowls. Garnish with something green, like parsley, chervil, chives or lovage.