In a mixing bowl, whisk the hot water, fish sauce, sugar, and oil together.
To make the marinade, trim the lemongrass stalks, leaving about 6 inches from the bottom. Peel the outer leaves and use only the tender bulb. Slice them into rings first then use a very sharp knife to finely mince.
Chop the shallots and scallions coarsely.
Add the lemongrass, shallots, scallion, and pepper to the liquid mixture and whisk to combine.
Place the slab of ribs with the meat side facing the bottom of a baking pan and pour the marinade over it. Cover and let it marinade overnight or at least 8 hours in the refrigerator.
When ready to grill, remove the slab from the fridge while you prepare the grill. Set up the charcoal grill by placing coals on one side and lighting them. Rake the coals to create a flat even bed to create an even steady heat zone.
Once the coals have burned down to about 375 degrees F, place the meat over the coals to sear for a minute on each side.
Move the meat to the opposite side lined with heavy duty foil and cook for about 90-100 minutes, basting every now and then with the marinade and flipping it.
Let the ribs rest for 15 minutes before slicing between each ribs. Serve immediately with jasmine rice and your favorite vegetables.