Preheat a pot over medium heat. Add sliced mushrooms to the hot pan. Let cook for about 4 to 5 minutes. Don't worry about the mushrooms sticking. The next step will resolve any sticking.
Add low-sodium vegetable broth, diced tofu, chopped green onion, and sliced nori seaweed to the pot. Continue cooking uncovered over medium heat for another 4 to 5 minutes.
Add chopped baby bok choy (or kale) to the pot. Add a lid and continue cooking for another 1 to 2 minutes. Turn off the heat and let sit.
In a separate bowl, add miso paste and a tablespoon of water. Whisk until miso is smooth and thinned out so you don't get chunks of miso paste in the soup.
Stir in the thinned-out miso paste until well incorporated into the soup. Enjoy immediately.
Notes
Variation: If you're adding the optional rice noodles, cook the noodles according to package instructions. Spoon miso and mushroom soup over the cooked noodles just before serving. Avoid storing the soup broth with the cooked noodles, since the noodles will absorb the broth.