Add the oil to a large pot over medium-hight heat. When hot add in the mushrooms and sauté for about 5 minutes until they have softened and begin to brown. Remove them from the pot and set aside in a medium bowl.

Now return the pot back to the heat and add the water. Use your spoon to scrape up any bits that were stuck to the bottom. Bring to a boil then reduce to simmer. Whisk in the miso paste, then add the udon noodles. Cook for 1 - 2 minutes until the noodles are loosened. Add the bok choy, garlic, and mushrooms back into the pot along with the miso soup and the tofu. Heat for another 1 - 2 minutes until everything is heated through. Divide among bowls and garnish with green onions, sesame seeds, and hot sauce if using.