Cut onions into fine rings, perhaps halve large onions beforehand.
Form onion rings in a colander, sprinkle with salt, stand 15 min. Rinse thoroughly under cold running water and then dry on/with kitchen roll. Mix in a baking dish with the parsley and sumac, turn to the other side, toss a little. Sprinkle with additional sumac for decoration.
Goes great with all kinds of grilled lamb, but also with beef and pork.