Ingredients:

  • 3 large yellow onions, thinly sliced
  • 2 tablespoons unsalted butter
  • 1 tablespoon olive oil (for onions)
  • 1 teaspoon packed brown sugar (optional)
  • 1 teaspoon balsamic vinegar
  • 1/2 cup full-fat mayonnaise
  • 2 tablespoons prepared horseradish sauce
  • 1 teaspoon Dijon mustard
  • 1 teaspoon fresh lemon juice
  • 1/4 teaspoon freshly ground black pepper (for mayo)
  • 5 lbs sirloin or rump steak
  • 1 tablespoon olive oil (for steak)
  • Flaky sea salt, to taste
  • Coarsely ground black pepper, to taste
  • 8 slices thick-cut sourdough bread
  • 2 cups watercress or rocket (arugula)
  • 4 slices mature English Cheddar cheese (optional)

Instructions:

  1. Prepare the Onions: Heat butter and 1 tablespoon of oil in a large skillet over medium-low heat. Add sliced onions, salt, and sugar (if using). Cook slowly, stirring every 5–10 minutes, for 45–60 minutes until deeply browned and sweet. Stir in balsamic vinegar during the final 5 minutes. Keep warm or set aside.
  2. Mix the Horseradish Mayo: In a small bowl, whisk together mayonnaise, horseradish sauce, Dijon mustard, lemon juice, and pepper until fully combined and smooth. Taste and adjust seasoning.
  3. Slice and Season the Steak: Ensure the steak is very cold. Using a sharp knife, slice the steak against the grain into thin strips (about 1/8 inch thick). Toss the sliced steak lightly in a bowl with 1 tablespoon of olive oil, salt, and pepper.
  4. Sear the Steak: Heat a large frying pan or griddle over high heat until smoking hot. Sear the steak in small batches for only 60–90 seconds per side until nicely browned but still juicy. Set aside.
  5. Assemble the Sandwich: Lightly toast or grill the sourdough slices until golden. Spread a generous layer of horseradish mayo on both inner sides of the bread slices.
  6. Build the Sandwich: On the bottom slice, layer the warm, seared steak mixture. If using cheese, place it directly over the warm steak to soften slightly.
  7. Top and Finish: Pile a generous mound of the caramelised onions over the steak, followed by a handful of fresh watercress. Top with the second slice of bread. Slice cleanly on the diagonal and serve immediately.