In a large pot, add the chicken broth, chicken breasts, soy sauce, miso paste, and sesame oil. Bring the mixture to a boil over medium heat.
Once boiling, reduce the heat to low and cover. Cook the chicken breasts for about 15 minutes or until they are fully cooked (internal temperature should be 165°F/75°C). Remove the chicken from the pot and set it aside to rest for a few minutes.
In the same pot, add the ramen noodles to the broth and cook according to package instructions, usually about 4–5 minutes.
In a skillet over medium heat, sauté the sliced shiitake mushrooms until they are tender, about 5 minutes. Season with salt and pepper to taste.
After allowing the chicken to rest, shred it using two forks.
Divide the cooked ramen noodles into four bowls, ladle the broth and added chicken over them. Top with sautéed mushrooms, fresh spinach, sliced green onions, and soft-boiled eggs if desired.
Enjoy your homemade Chicken Ramen hot!