In a large pot, bring water to a boil. Add the udon noodles and cook according to package instructions, usually around 3-5 minutes for fresh noodles. Drain and set aside.

In the same pot, heat the sesame oil over medium heat. Add minced garlic and sauté for about 1 minute until fragrant. Then toss in the sliced mushrooms and sauté for another 5-7 minutes, until they're tender and lightly browned.

Pour in the vegetable broth and bring the mixture to a simmer. Scoop out some broth into a small bowl and whisk in the miso paste until smooth, then return this mixture to the pot, stirring to combine.

Gently fold the cooked udon noodles into the broth and let everything simmer together for about 2-3 minutes to meld the flavors.

Add soy sauce for depth of flavor. Taste and adjust seasoning as necessary, adding more soy sauce or miso if desired.

Ladle the soup into bowls and top with sliced green onions. Enjoy!