Soften the mushrooms: boil 1 cup of water. Place the dried mushrooms in a heatsafe bowl, cover with the boiling water, and set aside.
Chop the yellow onions and celery. Peel and chop the carrots and potato.
In a large soup pot over high heat, combine the vegetable broth, split peas, marjoram, onions, carrots, celery, and potatoes. Cover and bring to a boil.
Meanwhile, remove the mushrooms from the soaking liquid and discard any hard stems. Finely chop or puree and add them to the soup. Strain the soaking liquid to remove any sediment or dirt and add the clear liquid to the soup pot.
When the soup begins to boil, stir well, reduce the heat to a simmer, cover, and continue to cook for 30 minutes.
After the soup has simmered for about 30 minutes, stir it well. Maintain on low heat with occasional stirring and cook for another 20 minutes, until the split peas are very soft and the soup becomes “creamy” when stirred.
Remove some of the broth from the pot and use it to thin the miso paste. Stir in the thinned miso paste, salt, and black pepper.