I like to serve these sandwiches on a freshly baked Ciabatta bread so that the juices from the onions and the steak pools into the large porous holes of the bread. It makes the sandwich moist without being wet and serves to contain a lot of the juice that will flow out of the meat as you bite in to it.
The complete sandwich is three separate recipes. The steak (that’s this recipe), the onions, and the bread. Only the first two are necessary to make for the meal if you already have suitable bread.