I’ve tried this dish at a few snack bars in America, and it’s best served when you’re famished and exhausted from work, because it works wonders then.
Fry the onions in a little bit of olive oil until they are golden brown. Then season with the pepper and wine vinegar.
Drizzle the meat with olive oil, season with pepper & salt and sear in a hot frying pan. After a few min turn to the other side and sprinkle cheese over the steaks. In the meantime, preheat the baguette in the oven.
And now the whole thing is served: for this, the baguette is cut in half and one half is covered with the onions. Add the meat first, then the arugula and finish by spreading the whole thing with mustard. Finally, put the other half on top, cut into two or four pieces and enjoy.