This is a suimono, a clear soup--a genre of Japanese soup that is very simple, so it relies on fresh, high quality ingredients. The broth is made from kombu and eryngii (king oyster mushroom), seasoned simply, and garnished with a sprig of mitsuba, parsley, or cilantro as well as a pinch of lemon zest. You could switch it up with some other mushrooms if you wish, or add some thinly sliced pieces of something like carrot.
Cut the mushroom stem crosswise into eight 3/4-inch slices.
Fill a saucepan with the mushrooms slices, water, kombu, and sea salt. Place saucepan over medium-high heat. Just before the water beings to boil, remove the kombu.
Bring the water to a boil, add the soy sauce (or tamari), then reduce the heat to low and simmer for 5 minutes.
To serve: Arrange 2 slices of eryngii mushroom, a sprig of mitsuba/parsley/cilantro, and a pinch of lemon zest in each bowl. Ladle hot broth into each bowl and serve.