Marinate the steak

Pat the flank steak dry with kitchen paper. Mix the olive oil, Worcestershire sauce, garlic, Dijon mustard, salt, pepper and smoked paprika in a bowl.

Rub the marinade all over the steak and leave for at least 15 minutes. For more flavour, cover and chill for 1–2 hours, then bring back to room temperature before cooking.

Caramelise the onions

Melt the butter in a frying pan over a medium heat. Add the sliced onions and cook for 10–15 minutes, stirring often, until soft and golden.

Add the brown sugar and balsamic vinegar, then cook for another 2–3 minutes until sticky and glossy. Set aside.

Cook the steak

Heat a heavy frying pan, griddle pan or barbecue until very hot. Sear the flank steak for 3–4 minutes on each side for medium-rare, depending on thickness.

Flank steak is best served pink, juicy and thinly sliced.

Transfer to a board and rest for 8–10 minutes.

Slice the steak

Slice the steak thinly against the grain. This keeps the meat tender and easy to eat in the sandwich.

Toast the bread

Slice the rolls or baguette portions and toast the cut sides until golden.

Spread a generous layer of Fenton Farm Truffle Mayonnaise over both sides of the bread.

Build the sandwich

Layer the base with rocket, then pile on the sliced steak. Add the caramelised onions and cheese.

Place under a hot grill for 1–2 minutes if you want the cheese melted.

Finish with sliced cornichons or pickled onions if using, then close the sandwich.

Serve

Serve immediately while warm, with crisps, fries, pickles or a simple green salad.