Peel the onions, cook half cooked in lightly salted water. Then drain and cut in half. Brush a baking dish with the oil, layer half onions with the cut surface facing up. Salt and pepper. Wash and finely chop the parsley, sprinkle over the onions. Drip a little more oil on the onions, then bake everything in the preheated oven at around 150 degrees for about an hour.