Soak the dashima and shiitake mushrooms in a large pot with water. You can skip soaking if you don’t have time, but soaking maximizes the flavor of the broth.
Bring it to a boil over high heat and boil, covered. Remove the dashima. Reduce the heat to medium, and continue to boil. Turn the heat off. Let the broth cool.
Pour the broth through a strainer into a large bowl. Press the vegetables with a spoon or spatula to squeeze out any remaining broth.
Store the broth in the fridge.