In a medium saucepan, boil the soba noodles according to package directions, except stop cooking just shy of done. They will continue cooking a bit whilst you prepare the broth. The brand I use calls for four minutes cooking time, I stop at about three minutes. Drain the soba noodles in a strainer and transfer to a pre-warmed serving bowl. Make sure to use a generously sized bowl to assure everything fits.
In the same saucepan, combine the broth, soy sauce, toasted sesame seed oil, ginger, garlic and white pepper to taste; and bring to a low boil. Add the broccoli and mushrooms (if using) and cook until just tender. Add the cooked salmon to reheat it.
Add the salmon, broccoli and mushrooms to the soba noodles and then pour the hot broth over all. Garnish with green onions and add shichimi togarashi, or other chile seasonng to taste.