Thinly slice the mushrooms, fennel stalks, celery, carrots, onions, and cabbage and place in an 8-quart stockpot. Cut the rutabaga into large chunks and place in the stockpot. Add the remaining ingredients and cover with cold water.
Bring to a boil over high heat. As the stock approaches a boil, skim any impurities that rise to the top with a ladle. Lower the heat and simmer gently for 45 to 60 minutes.
Remove from the heat and cool for 10 to 15 minutes. Ladle through a fine strainer, cool, and refrigerate.
To intensify the flavor, reduce the stock after straining by one quarter to one half.