Bring a small saucepan of water to a boil. Gently lower in the eggs and cook for 7 minutes for jammy yolks, or 9 minutes for firmer yolks. Transfer to cold water, peel, and halve.

Pat the chicken dry and season with the salt and pepper. Heat the neutral oil in a Dutch oven over medium-high heat and brown the chicken for 3 minutes per side.

Add the chicken broth, soy sauce, mirin, ginger, and garlic. Bring to a gentle simmer, cover, and cook until the thickest part of each thigh reaches 165°F (74°C), about 12 to 18 minutes.

Transfer the chicken to a plate. Stir the optional miso into a ladleful of hot broth until smooth, then return it to the pot. Taste the broth and adjust seasoning.

Cook the ramen noodles in a separate pot according to the package directions. Drain well; separate cooking keeps the broth clear and the noodles springy.

Slice or shred the chicken. Add the spinach to the hot broth for 1 minute, just until wilted.

Divide noodles among four bowls. Ladle over broth and spinach, then top with chicken, egg, scallions, corn, and nori. Serve hot.