Caramelise the Onions: Place a large frying pan over a medium-low heat. Add the butter and olive oil. Once the butter has melted, add the sliced onions and a good pinch of salt. Cook slowly for 20-25 minutes, stirring occasionally, until the onions are very soft, sweet, and a deep golden brown. For a detailed guide on this technique, BBC Good Food has an excellent explanation on how to caramelise onions properly.

Finish the Onions: Once the onions are nearly done, sprinkle over the brown sugar and stir. Cook for another minute before adding the balsamic vinegar. Let it bubble and reduce until the onions are sticky and jammy. Scrape the onions out of the pan into a bowl and set aside.

Prepare the Sauce and Bread: While the onions are cooking, you can prepare the sauce. In a small bowl, whisk together the mayonnaise, creamed horseradish, Dijon mustard, and garlic powder. Season with a little black pepper. Slice your ciabatta loaf horizontally and then into four equal portions.

Cook the Beef: Pat the thinly sliced steak dry with a paper towel and season generously with salt and pepper. Turn the heat under the frying pan up to high. Once the pan is very hot, add the beef in a single layer (you may need to do this in two batches to avoid overcrowding). Cook for 1-2 minutes per side until nicely browned but still pink in the middle. What works best for me is to move it around as little as possible to get a great sear.

Rest the Beef: Remove the cooked beef from the pan and place it on a cutting board. Let it rest for a couple of minutes. This is a crucial step to ensure the meat stays juicy.

Assemble the Sandwiches: Spread the sauce generously over both sides of the sliced ciabatta. Pile the cooked beef onto the bottom halves of the bread, followed by a generous layer of the caramelised onions.

Melt the Cheese: Top each sandwich with two slices of Provolone cheese. Place the top half of the ciabatta on. You can either place the assembled sandwiches under a hot grill for 2-3 minutes or return them to the warm pan, cover with a lid, and heat for 3-4 minutes until the cheese gets gloriously melty and the bread is warmed through.

Serve Immediately: Slice the beef sandwiches in half and serve straight away while they are warm and gooey.