While preparing the soup, cook the soba noodles according to their package instructions and set to the side when finished. (To prevent sticking, toss with a teaspoon of sesame oil or simply rinse under warm water in the sink once in a while if oil free.)
To a large stock pot over medium heat, add sliced onions, shitake mushrooms and a sprinkle of salt. Cook for 7 minutes until the onions are soft, stirring occasionally and adding splashes of water, broth or tamari to prevent sticking. Add minced ginger to the pot and stir.
Add the water, corn and carrots to the pot and bring to a simmer. Once simmering, cook for 5 minutes or until the carrots are just cooked.
While that is cooking, grab a small bowl and whisk together the sesame oil, tamari, rice vinegar, tahini, sriracha and miso until it's smooth and no chunks remain. Use a little of the soup base to thin if needed.
Remove the soup from the heat and gently stir in the tofu, miso mixture and spinach. Taste broth and adjust salt to taste.
To serve- Add a portion of soba noodles to a large bowl and spoon over the broth and veggies. Top with scallions, sesame seeds and more sriracha if you like!