Wash the pork slab with running water and make sure that hairs on the skin are shaved.
In a casserole, put 5 cups of water then add lemon grass, leeks, bay leaves, onion, black peppercorn, garlic, and salt.
Make the brine/marinade: Bring water to boil then lower heat and simmer for at least 10 minutes. This will allow the herbs and spices to release its flavor and aroma.
Turn off heat. Set aside brine solution until it cools.
Pour the brine on the pork slab and marinate overnight
After marinating, remove the pork from the brine. The surface of the pork is quite salty so we have to rinse with fresh water then pat dry with paper towel.
Lay the pork slab on flat surface and place the rolled lemon grass, leek, and garlic.
Now roll up the pork slab, form a roulade with the garlic, leeks, and lemon grass on the center. Tie pork with twine (string) to keep its roulade in shape.
Brush the surface with mixture of milk and water. Set aside for at least an hour to allow milk to set in the skin. Milk makes the skin color reddish brown when roasted.
Set and preheat your turbo broiler (or oven) at 375 deg F (190 deg C). Maintain this temperature up to end of cooking.
Put the pork roulade on the turbo broiler but cover with aluminum foil. I put aluminum to cover the meat temporarily for the first 2 hours of cooking to shield the hot air from the oven fan otherwise the outer part of the pork will be cooked quickly yet the inner part is still half cooked.
After 2 hours remove the aluminum foil cover. Continue cooking but rotate the roulade, at least every 10 minutes to evenly cook the skin. Check if the skin is already crackling crispy then it is done.