This easy stock will keep refrigerated for three days or frozen for two months.
Combine mushrooms and 2 cups water in a bowl; cover and let stand 20 minutes. Drain mushrooms in a colander over a bowl, reserving liquid. Rinse mushrooms. Place in a 3-quart saucepan; strain reserved liquid into the pan. Add 3 cups water, red onion, and remaining ingredients.
Bring to a boil over medium heat; reduce heat, and simmer, partially covered, 1 hour. Strain.