Preparation:

• Preheat the oven to 200°C.
• Heat a heavy cast-iron pan over high heat and season the steak well with salt and pepper.
• When the pan is good and hot, drizzle the steak with 1 tablespoon of the olive oil and sear in the hot pan. Brown the steak evenly for 2–3 minutes, turning once or twice.
• Transfer to the oven and cook for about 10 minutes (for medium), turning several times.
• Transfer the steak to a cutting board and let it rest at least 15 minutes (keep the pan to hand).
• Meanwhile, combine the mustard, anchovies and lemon juice in a blender. With the motor running, stream in 5 tablespoons of olive oil, then gradually add the Parmesan.
• Add 1 tablespoon cold water and blend until dressing is smooth and thick. Taste and season with salt and pepper.
• Heat the pan again, drizzle with more olive oil and toast the ciabatta until golden brown and crisp on both sides.
• Drizzle the ciabatta halves with a third of the dressing. Top with the rocket leaves and sliced onion and drizzle with half of the remaining dressing.
• Slice steak thinly and add to the sandwich. Drizzle with the remaining dressing, then cut the sandwiches crosswise into pieces and serve. 

Tip:

Make it a double decker! The sandwich can also be served folded, cut into pieces ­– and is then also suitable for a picnic.