Brown the mushrooms before adding broth
Let the mushrooms sit against the hot surface between stirs. They do not need to become crisp, but several browned faces will make the broth more rounded. Ginger and garlic follow near the end of this stage so their fine pieces spend less time on the hot base and are less likely to scorch.
Cook noodles in a separate saucepan
A separate pot keeps loose noodle starch out of the measured broth. Choose thin dried wheat noodles with a package cooking time near five minutes, and follow the package if it specifies a different interval. Keep them tender but distinct. Very wide noodles or dense dried udon would need a different timing allowance.
Drain and divide the noodles among four serving bowls just before the soup is ready. Letting a drained bundle sit for a long time can make it difficult to separate, so coordinate the two pots. If preparing ahead, keep cooked noodles apart from the broth and chill both promptly rather than leaving noodles to soak and swell.
Dissolve the miso off the heat
White miso is mixed with a ladleful of hot broth in a small bowl before being stirred back into the pot. This makes it easier to remove lumps without chasing them between tofu cubes. The recipe uses low-sodium vegetable stock because miso and soy sauce already contribute salt; check their labels before substituting.
Add the bok choy and firm tofu shortly before the broth finishes so the greens soften and the cubes heat through without being broken by prolonged stirring. Once the miso is incorporated, ladle the broth over the waiting noodles, sharing the mushrooms, tofu and greens among the bowls. Add the sliced scallions just before serving.
