Pull the chicken meat from the chicken and set aside.
If making jammy eggs: Bring a small pot of water to a boil. Lower to a simmer. Add the eggs, making sure they are fully submerged in the water, and cook for exactly 8 ½ minutes. Rinse under cold water to stop cooking. Peel and cut in half lengthwise. Set aside.
In a medium-sized pot, heat the stock, water, and chunk of ginger to a boil. Lower to a simmer.
In a separate pot, bring a pot of salted water to a boil. Add the noodles and cook according to the package directions. Drain, rinse, and divide among the bowls along with the nori sheets, if using.
While the noodles cook: In a little bowl or cup, make a slurry with the miso paste and a tablespoon or two of water, whisking with a fork until smooth. Add to the simmering stock, along with the soy sauce, mushrooms, spinach, and chicken. Simmer until warmed through and the mushrooms and spinach soften and wilt, about 3 minutes. Taste for seasoning.
Ladle the miso soup among the noodle bowls. Arrange a jammy egg on top, along with a generous amount of scallions. Serve immediately, with togarashi seasoning if you'd like a little spice.
