Add water to a large saucepan over medium-high heat. Stir in the soy sauce, Hondashi powder, mirin, and sugar until the powder and sugar dissolve. Bring to a boil then reduce to a simmer.

Add the udon noodles to the broth, cover with a lid, and simmer for 3-4 minutes or until the noodles soften and separate.

Add the mushrooms, cabbage, and half of the green onion. Raise the heat to medium and cook for 3-4 more minutes or until the vegetables soften.

Ladle into bowls and add a few spoonfuls of the silken tofu to each dish, topping with more broth to warm the tofu. Add more green onion, furikake rice seasoning, and togarashi seasoning, if desired.