Cook the onions in a little olive oil until golden brown, season with freshly ground black pepper and add a splash of white wine vinegar.
Drizzle olive oil over the thin slices of meat and season with sea salt and freshly ground black pepper. Cook the steaks on a very hot griddle pan, and after a couple of minutes turn them over and shave the cheese onto each steak. Warm the baguette in the oven.
To serve, slice the baguette in half, pile on the fried onion, then the steaks, top with rocket and finally spoon on the mustard. Place the top on the bread and slice into 4 or 2 pieces.