Soy sauce, mixed with garlic, chilli and sweetener - syn free
Or teryaki - Soy Sauce, Honey, ginger, lemon juice and garlic (need to count the syns for the honey, sorry can't remember the exact value but 1tbsp is not too bad)
I just made a Thai stir fry today, from a SW cookbook. I used a chopped red chilli, some grated ginger, chopped coriander, a splash of fish sauce, a little bit of water (recipe said stock but for a couple of spoonfuls....) and the best discovery: Swartzch Thai Seven Spice stir fry seasoning. It comes in a little spice jar and a little spinkle goes a long way. I got mine in Tesco. You can add whatever meat and veg you like, we had chicken tonight but have made it with pork before.