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1
Cook udon in a medium pot of boiling salted water according to package directions. Transfer udon to a colander to drain; reserve cooking liquid in pot. Transfer udon to a large bowl and toss with 1 Tbsp. oil.
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2
If using eggs, cover pot and return cooking liquid to a boil. Add eggs and cook at a low boil until soft-boiled, about 6 minutes. Transfer eggs to a large bowl of ice water to cool, then peel and reserve.
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3
Heat remaining 2 Tbsp. oil in a large pot over medium-high. Saute mushrooms and 1/4 tsp. salt until lightly browned and moisture releases, about 5 minutes. Stir in garlic and ginger; cook until fragrant, 30-60 seconds. Stir in miso, then add chicken broth. Stir in kombu, if using, soy sauce, and remaining 1/2 tsp. salt. Cover, bring to a simmer, and cook 10 minutes.
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4
Add meatballs and carrots. Cover and continue to simmer until meatballs are just cooked through, about 10 minutes. Remove kombu from broth and discard.
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5
Stir in bok choy and vinegar, if using. Place fish on top of stew, then cover and cook 3 minutes. Gently fold in shrimp (try to avoid breaking up fish) and cook, covered, until shrimp is pink and fish is opaque and cooked through, about 3 minutes more.
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6
Divide stew among bowls. Slice eggs in half lengthwise and top each bowl with 2 halves. Sprinkle with scallions. Serve udon in large bowl for sharing alongside.
