Place pork belly, chunks of onion, garlic cloves, leeks, bay leaves, and peppercorn in the pot.
Pour makgeolli and soy sauce all over the pork and aromatics, then bring it to a boil.
Once it starts boiling, start the timer. Let the pork belly boil on high heat for 20 minutes, uncovered. Flip the slabs, then let it boil on low heat for 30 minutes, covered.
Remove pork from the pot, let it sit for 10 minutes, then slice into 1/2-inch thick pieces.
Make a sauce by combining garlic, pork glaze, soybean paste or ssamjang, red pepper paste, sesame oil, and sesame seeds.
Plate the pork, serve with the sauce and spicy radish salad, and enjoy!