Sweat the thinly sliced onions in olive oil and butter at a low heat for about an hour. They must be soft and uncoloured. They will be considerably reduced. Allow them to cool. Butter a gratin dish. Beat the eggs with the cream. Add the onions. Bake in a medium oven for 40 minutes.
This is evidently tarte à l’oignon or zewelwai without pastry, which I initially omitted due to my dislike