Put the dried mushrooms in a large bowl, and pour the hot water over the mushrooms, making sure they are submerged. Let the mushrooms soak for at least an hour.
Then add the mushrooms and the water the mushrooms were soaking in to a blender. Blend on high until the mushrooms and water have made a very smooth mixture.
Pour the mushroom mixture into a small sauce pan. Then add the soy sauce or tamari, the agave, the miso, and the ginger to the pan, whisk to combine. Heat the pan on medium, bring to a simmer, reduce heat to low.
Simmer for about 10 minutes to cook the mushroom mixture and thicken slightly.
Now, in a small bowl, whisk together the cornstarch and water until the cornstarch has dissolved.
Whisk the cornstarch into the pan.
Then continue to simmer for about 5 minutes or until the oyster sauce is nice and thick.
Let cool and then keep in an air tight container and refrigerated. It will last for several weeks.
Notes
*I used a dried mushroom mixture with shiitakes, trumpet mushrooms and oyster mushrooms. Any kind will work though, but I recommend using shiitakes.