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Heat a griddle pan. Rub the steak with the olive oil, season, then cook for 2-4 mins on each side or until done to your liking. Put the steak on a plate to rest.
Put the roll halves cut-side down in the pan for 2 mins to soak up the meat juices and toast until golden brown. Spread the toasted side with the mustard and pile the watercress on top.
Cut the steak in half, sit on top of the watercress, then crumble over the stilton. Drizzle with a splash of balsamic vinegar, tip over any steak resting juices and serve.