Steak Sandwich with Caramelised Onions

Ingredients

• 2 sirloin steaks, about 200g each
• 2 onions, thinly sliced
• 20 g butter
• 1 tbsp olive oil
• 1 tsp balsamic vinegar
• 1 tsp soft brown sugar
• 2 ciabatta rolls, halved
• 2 tbsp Dijon mustard
• 2 tbsp mayonnaise
• 1 handful rocket
• salt and black pepper
• chunky chips, to serve
• dressed green salad, to serve
• cornichons, to serve

Method

1. Melt the butter with half the olive oil in a heavy frying pan over medium-low heat. Add the onions with a good pinch of salt — salt now pulls the water out and speeds the jamming. Cook slowly for 18–20 minutes, stirring often, until deep mahogany and sticky at the edges. Rush this and you get sweaty onions; take your time and you get jam.
2. Stir in the balsamic vinegar and brown sugar for the last 2 minutes — the splash of vinegar cuts the sweetness and lifts the whole pan. Tip the onions into a bowl and wipe the pan clean for the steaks.
3. While the onions go, pat the steaks bone-dry with kitchen paper — water means steam, and steam means no crust. Rub all over with the remaining olive oil and season generously with salt and plenty of black pepper. Leave them on the board to come up to room temperature.
4. Set the same pan back over the highest heat until it's smoking hard. Lay the steaks in well apart — don't crowd the pan, or the heat drops and they'll stew instead of sear. Cook 2–3 minutes per side for medium-rare, pressing for full contact. Pull at 52°C internal; the temperature carries up to 55°C as they rest. Lift onto a board and rest for 5 minutes — this is non-negotiable, or the juices end up on the board instead of in the meat.
5. Stir the Dijon and mayonnaise together in a small bowl until smooth. Taste it — it should be sharp enough to wake the steak up; adjust with a pinch of salt if it needs lifting.
6. Toast the ciabatta halves cut-side down in the still-hot pan for 1–2 minutes until golden and crisp — the residual beef fat seasons the bread on the way through.
7. Slice the rested steak across the grain into thick strips, spooning any board juices back over them.
8. Spread the mustard mayo generously on both cut sides of the ciabatta. Pile in the rocket, heap the caramelised onions on top, layer over the sliced steak with its juices, and press the lids down gently. Plate each sandwich with chunky chips, a dressed green salad, and a small pile of cornichons alongside — the sharp pickles cut through the richness of the steak and onions.