Onions & Parsley: Thinly slice the onions, or you can use the mandolin. Add the sliced onions in a bowl. Roughly chop the parsley leaves and add them in the bowl.
Mix the ingredients: Add lemon juice, sumac, olive oil, salt and pepper in the bowl. Mix all the ingredients so that the onion is coated with all the ingredients. Let it rest for 5 minutes before serving.