Stir and heat for a few minutes to obtain a coating sauce, without allowing it to become too thick. Add a knob of butter to a frying pan and quickly toss the onions in it.
Useful tips and interesting facts
Wrapping a piece of meat in bacon or strips of fat is called barding. Instead, larding means inserting fat, such as lard or bacon, into a large cut of meat. Both techniques are used to confer the right amount of fat to pieces of lean meat. Barding a fillet of beef will impart a naturally savoury flavour to the meat and help keep it compact even after cooking. Thanks to the bacon, this dish will be even more succulent.
The same dish may be prepared using pork fillet. In this case, however, the cooking times will be slightly longer because pork must be served well cooked.
Another way to prepare beef fillet is to cook it in the oven for 10-15 minutes at a temperature of 180°C. Instead of spring onions, you may use spinach, radicchio or any other type of leaf vegetable, while a more delicate flavour will be obtained if a quality Italian lardo is used as a substitute for bacon.