Place pork belly in a storage bag, add shio koji, and massage well. Remove air, seal the bag, and refrigerate for 12–24 hours.

Slice ginger and crush garlic to remove the core. In a pot, add the marinated pork, enough water to cover, sake, ginger, garlic, and green onion (green part). Heat on medium until boiling, then reduce to low and cover with thick cooking paper. Simmer for 30–40 minutes.

Turn off the heat, cover the pot, and let it rest for 30 minutes to cool slightly. Remove the pork, drain, and slice thinly.

Trim the ends of the cucumber and slice thinly with a peeler. Soak in cold water and drain. Cut the white part of the green onion into thin strips (shiraga negi) and soak in water. Finely chop the remaining green onion.

Mix the finely chopped green onion with soy sauce, sugar, vinegar, sesame oil, and doubanjiang to make the sauce.