For the onion gratin, peel the onions and cut in half lengthwise. Cut into half rings that are not too narrow. Put them in boiling water for 5 minutes. Drain very well. Cream the butter.
Gradually add the eggs, fold in the Parmesan cheese and chopped chives. Season well with salt, pepper and oregano. Grease a baking dish and sprinkle with breadcrumbs.
Put the onions in and pour the egg mixture over them. Cook the casserole in a preheated oven at 180 °C for about 20 minutes.