Bring a large saucepan of water to boil over high heat. Add noodles then cook until just tender, about 4 minutes. Drain and set aside.
Return saucepan to stove and reduce heat to medium. Add chicken stock, water and ginger then bring liquid to a boil. Reduce heat to medium-low and add mushrooms. Simmer mushrooms in broth for 10–12 minutes until softened.
Reduce heat to low then whisk in miso. Do not allow the liquid to boil once miso has been added. Taste for seasoning then add additional miso or a bit of soy sauce if broth lacks salt. If broth is too salty, just add a bit of water to dilute.
Add spinach and green onions then cook just until wilted and bright green. If adding items such as cooked chicken or tofu to soup, add them at this point and allow just enough time for them to warm.
Add noodles to bottom of serving bowl, ladle miso broth over noodles and add hardboiled egg. Serve while nice and hot.
One Last Thing
Soba noodles are made from buckwheat and taste amazing!