Place the pork belly in a big pot and fill with water up to the meat level. Add the salt, bay leaves, whole peppercorns and the halved onion. Bring to a boil and simmer on low for 40 minutes or until meat is tender.
Remove pork from the water and let rest on a rack. Transfer to a broiler-safe pan and broil in the oven for about 5-10 minutes or just until you see some charred spots on the meat. Turn the meat halfway for even browning. You can also grill it if you prefer. Watch carefully because fat burns easily.
Chop the pork into small cubes and set aside.
In a large pot, melt butter. Saute ginger and garlic then add the chopped liver. Mash the liver with the back of a wooden spoon or a potato masher until it’s no longer pink and resembles cooked ground beef (about 3-4 minutes).
Add the pork belly cubes and stir-fry for 5 minutes. Put the soy sauce, lemon juice (or vinegar), red pepper flakes and liquid seasoning (if using). Continue cooking until fat is rendered . Adjust seasoning.
Turn off heat. Add mayonnaise and chopped onion. Stir to combine and serve immediately with lemon slices and top with a sunny-side-up egg, if desired.