If adding the optional eggs, boil them for 6-8 minutes or to your preferred doneness. Place the cooked eggs in ice-cold water to stop them cooking further. Then peel them and slice them in half lengthwise. If they turn cold, they will heat up again in the ramen broth.
In a mixing bowl, whisk all the marinade ingredients together and stir in the chicken. Allow to marinate for up to four hours or while you fry the onions.
Heat a tablespoon of sesame oil in a large saucepan over medium heat. Stir in the finely chopped onion and fry for about 5 minutes to soften. Add the marinated chicken with all the marinade and fry for about 3 minutes, stirring regularly until it is about 80 percent cooked through. Add in the enoki mushrooms.
Pour in the stock and bring to a simmer over a medium-high heat. While the stock is coming to a simmer, begin soaking your ramen noodles as per the packet instructions. Be sure not to let them soak too long or they will fall apart and become mushy. You want them to be a bit al dente as they will continue to cook in the hot broth. Taste and adjust seasoning.
Divide the noodles between 4 bowls. Top with chicken and broth and add in eggs if using. Garnish with spring onions and whatever else you choose.