Reconstitute the dried shiitake mushrooms overnight in a refrigerator, so that they are nice and plump. (You will be using the soaking liquid later)
Cut off the stems of the reconstituted shiitake mushrooms. Put the mushrooms in a pot, along with their soaking liquid and the dashi bag. Bring to a boil before cooking for two minutes on medium heat.
Add the sugar, mirin, and soy sauce. Simmer slowly and allow the liquid to thicken, until less than half of the initial amount of liquid remains. Add the seasoning and cook for 10 more minutes. Remove the dashi bag.
For sushi, go a little bit heavier on the seasoning than you otherwise would, in proportion to the remaining amount of broth.
Prepare the sushi rice. Roll the sushi rice into bite-sized pieces.
Cover the bottoms of the stewed shiitake mushrooms with wasabi. Place a piece of shiso leaf on top of the sushi rice, and then the mushroom cap to top it off. Decorate with your choice of garnishes (microgreens, etc.).