Slice the mushrooms, fennel stalks, celery, carrots, onions, and cabbage thin 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. Ladle through a fine stainer, cool, and refrigerate. The stock will keep for 4 to 5 days in the refrigerator, or freeze in 1-cup batches.
To intensify the flavor, reduce the stock after reducing by one quarter to one half.