I have long ordered som tum without sugar, as I do with all Thai food. Sometimes they look at me strangely and say "we never put sugar in this dish". I just smile and say "good". Ordering som tum with out fish sauce is just as easy. I also order it with one chili, "neung met". The lady in my local som tum stand has always remembered that I like only one chili and no sugar ever since the first time I ever asked her and always repeats it to me. I do like larb gai very much.