Chop parsley. Thinly slice onions. In a big mixing bowl,take onion and 1/2 tbsp of salt, rub onion and salt with your hands until onion gives its juice out. Wash and drain. Squeeze onion to get rid of all the juice. Take them in another large mixing bowl. Add chopped parsley, sumac, salt, and pepper in the bowl. When you are ready to eat, drizzle lemon juice all over.