Pull the fat off the sirloin, finely slice the fat and place it in a large cold non-stick frying pan.
Put on a medium-high heat to render as it heats up, moving it around to coat the pan, while you peel and slice the onion into 1cm-thick rounds.
Add them to the pan to char for 10 minutes, turning halfway.
Meanwhile, cut off the sinew, then place the steak between two sheets of greaseproof paper and pound with your fist until just under 1cm thick.
Sprinkle with a pinch of sea salt and black pepper, then brush all over with the mustard and cut into two.
Add a good splash of red wine vinegar to the onion, toss for 1 minute over the heat, then divide between two slices of bread, leaving the pan on the heat.
Sear the steaks in the screaming hot pan for just 40 seconds on each side, then slice and lay over the cheese, cover, turn the heat off and leave to melt for just 40 seconds more.
Lay the steak on top of the onion, pop the other slices of bread on top, drizzle with a little extra virgin olive oil, and devour.
