- 1/2 cup dried sardines (about 1 ounce), or 1 (5″) piece dried kombu
- 5 ounces mature spinach (about 3/4 bunch), trimmed
- 3/4 teaspoon kosher salt, plus more
- 1/2 cup snow peas
- 2 boneless, skinless chicken thighs (about 8 ounces), cut into 1″ pieces
- 7 ounces thinly sliced shiitake, shimeji, or enoki mushrooms, roots trimmed
- 2 (1/2-ounce) pieces fried tofu (aburaage), cut into 1/4″ slices (optional)
- 3 tablespoons soy sauce
- 1 teaspoon mirin
- 8 ounces udon noodles
- 1/2 cup sliced scallions
- 1 1/2 ounces Japanese fish cake, thinly sliced (optional)
- 2 large eggs
- Place sardines or kombu in a large Dutch oven or donabe, fill with 6 cups water, and set aside at least 15 minutes or up to 1 hour.
- Meanwhile, cook spinach in a medium pot of boiling salted water until just cooked through, about 1 minute. Using a slotted spoon, transfer to paper towels. Gently squeeze water from spinach, then transfer to a cutting board and cut into 2″ slices. Transfer to a plate. Return liquid to a boil, add snow peas, and cook until just cooked through, about 1 minute. Drain and transfer to plate with spinach. Wipe out pot and reserve.
- Bring sardine liquid to a simmer over medium-high heat. Using slotted spoon, discard sardines, then reduce heat to medium. Add chicken and simmer until cooked through, about 5 minutes. Add mushrooms, tofu (if using), soy sauce, mirin, and remaining 3/4 tsp. salt. Cook, stirring, until mushrooms are tender, 3–5 minutes.
- Meanwhile, cook udon in reserved pot 2 minutes less than package directions.
- Bring chicken mixture to a simmer. Drain noodles and add immediately to pot. Add spinach, snow peas, scallions, and fish cake (if using), arranging each into sections and leaving an opening for egg.
- Gently beat eggs in a small bowl with a whisk or chopsticks. Pour into broth and cook until just set, about 2 minutes.
- Serve soup immediately in pot tableside, portioning into individual bowls.