Start by preparing the broth. In a large pot add water, mushrooms, kombu, shiitake mushroom, onion, green onions, carrot and leek. Bring the mixture to a boil then lower it to a simmer. Cover and let cook for 30 minutes then strain and set aside. Reserve the carrots.
Season the broth to taste by adding tamari and sesame oil.
Blanch snow peas, spinach and silken tofu and set aside. Prepare soba noodles according to package directions.
To assemble, add noodles to the broth and the remaining toppings. Garnish with sesame seeds, togarashi, nori and green onion if desired.