Bruise lemon grass by hitting it in several places with the dull side of a knife.
Mix all marinade ingredients. Reserve 1/2 cup for basting.
Place ribs in a very large zipper storage/freezer bag (2 gallon size) -OR- place each rack in a 1 gallon bag (you may have to cut racks in half. I often do just that because the huge zipper bags are hard to find where I live.
Pour marinade over ribs, seal bag(s) and "mush" the bag to distribute the marinade.
Place in refrigerator to marinate overnight.
The ribs are prepared by indirect cooking. With a gas grill (2+ burners) this means the ribs cook on the UNLIT side with the lid closed.
Preheat the grill to 300-325 degrees F--I put my left burner on medium heat then close the lid. My grill also has a built-in temperature gauge, if yours doesn't, an oven thermometer will do.
Remove ribs from plastic bag(s). Discard the marinade in the bag(s). Place ribs on UNLIT side of grill, close lid.
Roast low & slow for 3 hours, basting every 30 minutes with reserved marinade.
OVEN METHOD--Preheat oven to 300 degrees F. Place ribs on a rack in a roasting pan and proceed as in grill method.