Rinse the pork bones thoroughly under cold water to remove any impurities. Then, place them in a large pot and fill it with enough water to cover the bones. Bring the water to a boil and cook for 5 minutes. Drain the bones and rinse well again.

In the same pot, heat some oil over medium heat. Add the sliced onion, minced garlic, and ginger. Sauté until fragrant and the onion becomes translucent.

Add the pork bones back into the pot, along with gochugaru, gochujang, soy sauce, sesame oil, and sugar. Stir well to coat the bones and spices evenly.

Pour in the water, cover the pot, and simmer over low heat for about 2 hours, or until the pork bones become tender and the flavors meld together.

Once the bones are tender, add the potatoes, cabbage, and Korean radish to the pot. Stir gently to combine. Continue simmering for another 20-30 minutes until the vegetables are cooked through and the soup has thickened slightly.

Season with salt and pepper to taste. Adjust the seasoning if needed.

Serve the gamjatang hot, garnished with chopped green onions. You can enjoy it as is or serve it with a side of steamed rice for a hearty meal.