Blanch the bones: Place pork bones in a large pot, cover with cold water, and bring to a rolling boil for 5–7 minutes. Drain and rinse the bones under cold water to remove scum.
Rinse the pot.
Start the broth: Return the bones to the pot with 6–8 cups of water, the onion, ginger, 3 whole garlic cloves, peppercorns, and scallion pieces. Bring to a boil, then lower to a gentle simmer. Cook uncovered for 60–90 minutes until the broth looks milky and the meat is tender.
Make the seasoning paste: In a bowl, mix gochugaru, gochujang, doenjang, soy sauce, minced garlic, fish sauce (if using), perilla seed powder, and sesame oil to form a thick paste.
Skim and season: Skim off excess fat from the broth.
Remove and discard the onion, ginger, and peppercorns. Stir in the seasoning paste until dissolved.
Add vegetables: Add potatoes and napa cabbage. Simmer 15–20 minutes, until the potatoes are fork-tender and the cabbage is soft.
Finish and adjust: Stir in perilla leaves.
Taste and adjust with salt, a splash of soy sauce, or more gochugaru for heat. The broth should be savory, slightly nutty, and pleasantly spicy.
Serve hot: Ladle into bowls with plenty of broth, meat, potatoes, and cabbage. Serve with steamed rice on the side.