Caramelise the Onions: Heat 1 tbsp of olive oil and the butter in a large frying pan over a low-medium heat. Add the sliced onions and a good pinch of salt. Cook slowly for 15-20 minutes, stirring occasionally, until they are soft, golden, and sweet. Don't rush this part!

Finish the Onions: Once the onions are deeply golden, sprinkle over the brown sugar and stir for another minute. Add the balsamic vinegar, scraping up any browned bits from the bottom of the pan. Cook for another 2 minutes until the vinegar has reduced and the onions are jammy. Set aside and keep warm.

Prepare the Sauce and Bread: While the onions cook, mix the crème fraîche, horseradish sauce, lemon juice, and a pinch of salt in a small bowl. Set aside. In another small bowl, mix the remaining 1 tbsp of olive oil with the minced garlic.

Cook the Steak: Pat your steaks completely dry with a paper towel and season them very generously on all sides with salt and pepper. Heat a heavy-based frying pan or grill pan over a high heat until it's smoking hot. Add the steaks to the dry pan.

Sear and Rest: Cook the steaks for 2-3 minutes per side for medium-rare, or until cooked to your liking. For a detailed guide on timings and temperatures, you can check resources like Serious Eats for searing techniques. Once cooked, transfer the steaks to a cutting board and let them rest for at least 5-10 minutes. This is non-negotiable for a juicy steak!

Toast the Bread: While the steak is resting, brush the cut sides of your ciabatta rolls with the garlic oil. Place them cut-side down in the hot steak pan for 1-2 minutes until toasted and lightly charred. You can also do this under a grill.

Slice the Steak: After resting, slice the steak thinly against the grain. You'll see the lines of the muscle fibres; you want to cut across them. This makes the meat incredibly tender to eat.

Assemble Your Sandwich: Spread a generous layer of the horseradish cream on the bottom half of each toasted roll. Top with a handful of rocket, a pile of the warm caramelised onions, and finally, the sliced steak. Place the top half of the roll on and press down gently. Serve immediately.