If mushrooms are not to your liking, they can be omitted; reduce olive oil to 1 tablespoon.
In a large stockpot over medium heat, lightly cook onion, carrots, celery, scallions, and garlic in olive oil until onions become translucent. Add mushrooms and cook until they begin to release their juices (about 5 minutes). Add the remaining ingredients and bring to a boil, then reduce heat and simmer 1-½ hours.
Remove the stock from the heat, then strain the liquid through a fine strainer. Let the stock come to room temperature, then cover and refrigerate. Use or freeze extra.