Fillet Steak on a Bed of Sweet and Sour Onions

I think the first time I had fillet steak must have been outside Spain because this cut of meat had always been way beyond my mother’s food budget. This is a recipe that combines onion, which is such an essential part of Spanish cookery, with the sweet and sour flavours we have inherited from our Moorish forebears. The meat cooks beautifully: browned on the outside but rare and succulent inside.