How to Make It
Heat 2 teaspoons vegetable oil in a large saucepan over medium-high. Add mushrooms and cook until beginning to brown, about 4 minutes. Remove from pot.
Reduce heat to medium and add remaining 2 teaspoons oil to pot. Add scallion whites, ginger, and garlic. Cook, stirring, until fragrant, about 2 minutes. Add broth and bring to a simmer.
Remove a ladleful of broth from the pot and transfer to a small bowl; add miso and whisk until smooth. Return to pot; add tamari and sesame oil.
Lower eggs into a large pot of gently boiling water; cook for 6 minutes. Using a slotted spoon, transfer eggs to a bowl of ice water. Set aside.
Return water to a boil. Add noodles and bok choy; cook until noodles are al dente and bok choy turns bright green, 2 to 3 minutes. Drain; remove bok choy. Rinse noodles under cold water, then drain again. Divide noodles and bok choy among bowls.
Peel eggs and halve lengthwise.
Pour broth over noodles. Top with mushrooms, eggs, and scallion greens. Roughly chop sesame seeds and red pepper flakes; sprinkle over bowls. Crumble nori over each bowl, if desired.