How to Make It
Add the water, soy sauce, mirin, ginger, garlic and scallions to a saucepan. Place over medium heat and bring to a boil. Remove from heat and allow the liquid to cool to room temperature. Place in refrigerator for at least two hours.
Boil the eggs in a large pot of boiling water for exactly 6 minutes and 30 seconds. While the eggs are cooking prepare an ice bath. When ready, immediately transfer the eggs to the ice bath and allow them to cool for at least 5 minutes. Remove the eggs and peel.
Add the eggs to the marinade and allow to steep for at least 6 hours or overnight.
Add the chicken stock, rice vinegar, soy sauce and sugar to a medium stock pot and bring it to a boil over medium heat. Remove from heat and allow the liquid to cool to room temperature. Add the sesame oil and karashi mustard and mix well. Cool the sauce in the refrigerator for at least 2 hours. Mix again before serving.
Add the yuzu kosho, vegetable oil, salt and white pepper to a mixing bowl and thoroughly combine. Add the chicken thighs and coat with the mixture. Preheat a sauté pan over medium heat. When the pan is ready, add the chicken skin side down. Cook for about 5 minutes. Turn the chicken and cook for about 5 minutes more, until chicken is cooked through. Set the chicken aside and allow it to cool to room temperature. Slice the chicken after it cools and divide into 4 parts.
Prepare an ice bath. Place a large pot of water over high heat and bring to a boil. Add the ramen noodles and cook for 2 minutes. Strain the noodle and place in an ice bath for 1 minute. Gently swirl the noodles as they cool. Strain and divide into 4 bowls.
Pour 1 cup of the sauce on top of the noodles in each of the 4 bowls. Artfully arrange the chicken, egg halves, tomato, cucumber, scallions, mung bean sprouts and radish sprouts over the noodles in each bowl. Garnish with the black sesame seeds and serve.