Season and sear the steak

• Season the steak generously with salt and pepper.
• Heat olive oil in a frying pan over high heat.
• Sear the steak until deeply browned on both sides.

Rest and slice the steak

• Transfer the steak to a board and let it rest.
• Slice thinly against the grain.

Toast the baguette

• Split the baguette lengthwise.
• Toast the cut sides until crisp.

Assemble the sandwich

• Spread horseradish sauce on the baguette.
• Layer the sliced steak, onion, arugula and onion strings.
• Close the sandwich and serve.