Wash and clean the pork belly slab in running water. Drain.
In a medium size pot, prepare the brine by combining the water, lemon grass, leeks, bay leaves, pepper corns, garlic, sugar and salt.
Bring to a boil and simmer for about 5 minutes to release the aroma and flavor of the herbs and spices. Turn off heat and let it cool.
Put the pork belly slab and submerge in the flavored brine and place in a refrigerator. Let it soak overnight for best result.
After brining, wash the pork belly in running water and pat dry.
Place the pork belly in a flat surface and place in the center the stuffings: lemon grass, leeks, garlic and onion.
Roll the pork belly like a jelly roll and tie it with twine until it is secured and will not open up and spill the stuffing.
To make the lechon skin even more crispy and have a crackling skin, do this extra step: Poke a paring knife or fork on the skin. Rub salt all over the skin and meat then place the pork belly in a refrigerator to chill for few hours.
Place the pork belly in a roasting pan with a rack and brush it with milk/water mixture. Cover with aluminum foil.
Place in a preheated oven 190°C and roast for 2 hours. Then after 2 hours, remove the foil, brush again with milk/water mixture and continue roasting for another hour until the skin is crispy. Serve hot.
In case the skin is not yet crackling crispy, extend the cooking time to 45 minutes to 1 hour and raise the temperature to 220°C.