Season the steak with plenty of freshly ground black pepper and rub over the olive oil. Heat a pan until smoking, then add the steak and fry for 1-2 minutes on both sides (for rare), or until cooked to your liking. Remove from the pan and allow to rest on a warm plate.
For the onions, heat the olive oil together with the butter in a pan and fry the onions for 5-7 minutes, or until softened and golden-brown. Stir in the cream and mustards and continue to cook for 3-4 minutes, or until the mixture is thickened.
To serve, place one slice of the steak on a piece of toast, top with the onions and pancetta, finishing with the parsley. Place the second layer of toast on top and repeat the layer with the remaining steak, onions, pancetta and parsley. Top with the remaining piece of toast, press down, slice into two and serve.