Halve the onions lengthwise, then slice them thinly using a sharp knife. Transfer to a bowl. Add the salt and mix well.
Let sit for 20 minutes to allow the onions to soften. Squeeze out any excess liquid from the onions, then transfer them to a clean bowl.
Add the olive oil, lemon juice, sumac, sugar, and parsley.
You can serve the salad immediately for a fresh and sharp flavor. However, letting it sit for 20–30 minutes before serving allows the onions to soften slightly and the flavors to meld, resulting in a more balanced taste. For an even deeper flavor, refrigerate it for a few hours or overnight, but note that the onions will mellow, and the salad will lose some of its initial sharpness. Taste and adjust by adding more salt, sumac, or lemon juice, if desired.
The onion salad is kept in the refrigerator for up to 5 days.