Prep the mushrooms: Wash the shiitake mushrooms, remove and discard the tough stems, then thinly slice the caps.
Sauté the aromatics: Heat the oil in a large soup pot over medium heat.
Add the onion and mushrooms and cook for 3-5 minutes until their moisture evaporates.
Stir in the garlic and carrots and cook for 3 more minutes.
Simmer the soup: Add the water or stock, brown rice, marjoram, and thyme. Increase heat to bring to a boil, then reduce to low, cover, and simmer for 45 minutes or until the rice is tender.
Temper the miso: In a small bowl, whisk the miso with about 1/2 cup / 125 mL of the hot soup broth until it forms a smooth paste.
Finish and serve: Remove the soup from the heat.
Stir in the miso paste and fresh parsley.
Serve at once.