In a large heavy-bottom pot, layer the onion rings, apple slices, pear slices, leek, ginger, garlic, and black peppercorns across the bottom. Season with salt.
Combine the doenjang, soy sauce, and sugar into a rough paste and rub it all over the pork belly, covering every surface evenly.
Place the pork belly directly on top of the aromatics, skin side up if possible. Put the lid on the pot and make sure it seals well. Place over low heat and cook for about 1 hour, undisturbed.
Remove the pork belly from the pot and let it rest for a few minutes before slicing. Strain the liquid left in the bottom of the pot through a fine mesh strainer to get a clean, concentrated broth.
Slice the pork belly and serve with radish kimchi, salted cabbage, and whatever other sides you prefer.
