Cut the onion, potato, and pork into bite-sized pieces.
Heat some cooking oil in a pan and stir-fry the pork.
Add the onion and potato and stir-fry together.
Add black bean paste (Chunjang) and water, then simmer over medium heat.
When the potatoes are cooked, add starch water to thicken the sauce.
Pour the jajang sauce over boiled noodles to serve.