Adapted from Kitchn- How To Make Kombu Broth (Vegetarian Dashi)
Combine the kombu and water in a bowl. Soak kombu for 8 hours or overnight in the fridge.
Over medium-high heat bring the water with kombu to a simmer. Remove the kombu right before water comes to a boil. Add the shiitake mushrooms and simmer for 1 minute. Remove saucepan from heat and steep for 5 minutes. Remove the mushrooms (can slice and place into soup). Place the saucepan back on the heat. Add the tamari and mirin and bring to a boil. Strain the broth and set aside. Reheat before serving.
In a skillet, heat the oil/butter over medium heat. Add the nappa cabbage and stir fry until softened, about 5 minutes. Remove from heat and drizzle with sesame oil and season with sea salt.
Cook the soba according to the package instructions. Drain and then rinse in cold water.
Prepare the soup. Place the soba in bowls and cover with hot broth, cabbage, and green onions. Serve immediately!