Wash the pork slab with running water and make sure that the hairs on the skin are shaved.
In a pot, put in 5 cups of water, then add lemongrass, bay leaves, onion, black peppercorns, garlic, and salt.
Make the brine/marinade:
Bring water to a boil, then lower heat and simmer for at least 10 minutes. This will allow the herbs and spices to release their flavors and aromas.
Turn off heat. Set aside brine solution until it cools.
Pour brine onto the pork slab and marinate overnight (about 10-12 hours).
After marinating, remove the pork from the brine. The surface of the pork is quite salty, so we have to rinse with fresh water and then pat dry with a towel.
Lay the pork slab on a flat surface and place the rolled lemongrass, leek, and garlic onto it.
Now roll up the pork slab, form a roulade with the garlic, leeks, and lemongrass in the center. Tie pork with twine (string) to keep its roulade shape.
Brush the surface with a mixture of milk and water. Set aside for at least half to an hour to allow the milk to seep into the skin. Milk makes the skin color reddish-brown when roasted.