Photo: David Loftus
Remove the steak from the fridge and let it come to room temperature. Add the rosemary, then cover it with greaseproof paper and bash it with the base of a pan to make it all roughly the same thickness.
Peel the onions and remove the first slice of flesh, then slice into 2cm-thick rounds. Put the bay leaves, butter, 1 tsp olive oil and the sugar into a large non-stick frying pan on a medium heat. Once the butter has melted, place the onions in the pan in a single layer, then season and cook for 5 minutes. Pour in the vinegar, pop the lid on, reduce the heat to low and cook for 35 minutes, until beautifully golden and caramelised on the bottom only. Add splashes of water to loosen, if needed. Meanwhile, pop the ciabatta in the oven at 110ºC/fan 90ºC/gas mark ¼ and leave to warm through.
Season the steak all over. Place a large non-stick heavy-bottomed pan on a high heat. Once hot, drizzle the steak with the remaining olive oil, then cook for 3 minutes on each side for medium, or to your liking. Remove to a plate, rest for 1 minute, then slice 1cm thick and toss through its own resting juices and the extra virgin olive oil. Cut the warm ciabatta in half and rub the inside of each piece in the steak resting juices. Spread on the mustard, then layer over the caramelised onions, the steak and pinches of lemon-dressed watercress. Pop the other piece of bread on top, press down lightly and carve up into decent chunks to serve.
Jamie's Comfort Food by Jamie Oliver is out now (Michael Joseph, £30)