This is a pretty standard salsa recipe. I don't want to undermine the awesomeness of it, because it is truly great, but it's ultimately ... salsa. No big tricks, no low carb magic, just good solid ingredients.
Ok, I'm kind of lying. I just make these things up, as I go along. This salsa is AWESOME!
Use the best, freshest ingredients. Also, use a sharp knife when cutting your onions and garlic, so that they don't bleed and oxidize, giving that weird "old onion" taste. Fresh, sweet vibrant onions and garlic really help make this a tasty little salsa. I also DO somewhat cut back on the onions, because ... onions are pretty carby. There's just enough there, so that you know they're there, but without loading this with extra unnecessary carbs. For me, the big thing is fresh lime juice, a speckle of sweetener and enough salt to kick the lime juice's tarty nature ... right in the tail.