Place water, stock, or dashi in a pot over high. Whisk in miso paste and soy sauce. While waiting for broth to come up to a boil, chop spinach and cut tofu into small cubes.
When broth is at a boil, turn heat down slightly and add noodles. Cook for around 3 minutes or according to package directions. Stir in tofu and spinach until wilted, about 30 seconds. Stir, then taste to see if any salt is needed. Top with seaweed and scallions, if using.
