Heat your flat grill to medium and fry the onion in the oil. When the onion is cooked, add the thinly sliced beef and warm through, tossing with tongs.
Pour the veal stock over the top and add the parsley. Cook for a minute for the beef to absorb the stock and for it to reduce slightly.
Divide the mixture between the four bread rolls.
Season with salt and freshly ground black pepper.