In a large pot over medium-low heat, sauté the onion for 5 or 6 minutes, while stirring. Add the garlic, and sauté for another 2 minutes, while stirring. Add a splash of water if anything sticks.
Add the 16 cups of water, dried mushrooms, sprig of thyme, salt and pepper. Bring this to a simmer, and cook over low heat for 25-30 minutes.
Let your Broth cool completely, and then strain it into a freezer safe container, removing the mushrooms and herbs. Place the strained mushrooms and herbs into a clean dish towel, twist and squeeze the towel over your reserved Broth, to get every last drop of Broth, and then discard the mushrooms and herbs.
Store in the fridge for a week, or in the freezer for up to 6 months.