Combine all ingredients in a large pot over high heat; bring to a boil. Reduce heat to low and simmer, uncovered, for 4 to 5 hours. (The dried mushrooms will take up roughly one half of the liquid and you will lose a bit more to evaporation. For a lighter flavored stock and more of it, add water midway through to replenish to the original liquid level.)

Strain out solids with a colander set over another large pot or bowl. Pick out the mushrooms and garlic and reserve for another purpose.

Pour stock through a fine mesh sieve back into the first pot. Cool to tepid. Ladle into freezer-safe containers.