Set your slow cooker to Low and prepare your chicken thighs by trimming any excess fat.
In a small mixing bowl, combine soy sauce, miso paste, garlic, ginger, and sugar. Stir until smooth.
Pour the soy-miso mixture into the crockpot and add the chicken thighs, ensuring they are well coated.
Pour in chicken broth and water to reach halfway up the chicken or fill your crockpot to 3/4 capacity.
Add shiitake mushrooms and stir in sesame oil.
Cover and cook on Low for 6–8 hours, until the chicken is very tender and easy to shred.
About 30 minutes before your desired finish time, add the bok choy heads to the soup to preserve their crispness; cook until slightly tender but still bright and green.
Cook the ramen noodles separately according to the package instructions. Remove from heat and drain.
To serve, transfer ramen noodles to bowls and top with shredded chicken and some of the soup. Garnish each bowl with scallions, toasted sesame seeds, and optional hard-boiled eggs.
Drizzle with a teaspoon of rice vinegar just before serving for a fresh finish.