Note: if raw onion is too sharp for your taste, soak slices in cool water for 10 minutes, then drain.
Add onions to a jar or bowl. Add the lemon, olive oil, sumac, salt, and chile flakes if using. Mix, massaging gently. Stir in the parsley. Taste and adjust seasoning.
While the onions are ready to use immediately, if possible, let sit for at least 30 minutes to blend the flavors. Sumac Onion Salad is even better the next day and will keep for several days, although the parsley will deteriorate.
Find it online: https://frugalhausfrau.com/2025/09/14/sumac-onion-salad/