Cut onion, pork, and green onion into bite-sized pieces.
Add cooking oil to a pan, then stir-fry the green onion to make scallion oil.
Add the pork and stir-fry until cooked.
Add the onion and stir-fry until translucent.
Add chunjang (black bean paste) and sugar, then stir-fry evenly.
Pour in water and soy sauce, then bring to a boil.
Adjust thickness with starch water.
Pour the stir-fried sauce over cooked noodles to finish.