Wash and drain the pork belly slab. Remove hairs in the skin by using a shaving razor or sharp knife.
Fill the large pot with water just enough to cover the meat. Bring to a boil then add in lemon grass, garlic, black peppercorns, salt and bay leaves.
Put the pork belly slab into the pot and let it boil in medium heat for 1 to 1 & 1/2 hours or until the pork is tender.
Remove from pot and drain in a colander. Set aside and let it cool down.
Poke the skin with a fork and brush with vinegar then let it dry. Rub with fine salt the skin and the entire surface of the pork.
Heat cooking oil in a pot deep enough to fry the pork belly slab. Fry the pork until golden brown then remove from oil and drain in a colander. Let it cool for 10 minutes.
Then heat the oil again and re-fry the pork belly until golden brown and the skin is crispy.
Remove from oil and let it cool. Then slice the lechon kawali and serve with cucumber-vinegar dipping sauce.
To make the dipping sauce: Combine all the ingredients in a bowl and stir until the sugar is dissolved.