Soak the dried shiitake for at least 20 minutes in hot water and cut them into stripes afterwards. If you're using fresh shiitake mushrooms remove dirt with a brush or soft towel.
Add the shiitake and fry them for another 5 minutes.
Meanwhile, bringt 1 liter of water to a boil (e.g. in a water kettle).
Add ca. 5 tablespoons of miso paste to the mushrooms and onions and stir everything together. Now deglaze everything with the boiling water and stir until the miso paste has completely dissolved (which should happen relatively quickly).
Add the tofu, spring onions and seaweed to the soup and - if necessary - some more miso paste to taste.
Once the seaweed is welled and soft you can add the udon noodles and let it simmer until they are ready. As the noodles are pre-boiled they are done within minutes.
Serve in bowls and garnish with fried onions, chili sauce and herbs to taste.
Anmerkungen / Notes
*affiliate links. I'm giving you those link so that you know, what you're looking for. You can get all those ingredients in Asian supermarkets as well as (organic) supermarkets.
Recipe by Purple Avocado at https://purpleavocado.de/en/miso-soup-for-beginners/