
Slice the sweet red onions thinly. Heat 1 tbsp of olive oil and the butter in a pan over medium heat. Add the onions and a pinch of salt, cooking gently until they are soft and caramelised, about 15-20 minutes.
Preheat a griddle or frying pan to high heat. Season the sirloin steak generously with salt and black pepper. Add 1 tbsp olive oil to the pan and sear the steak for 3-4 minutes on each side for medium-rare. Adjust the cooking time for your preferred doneness. Allow the steak to rest for at least 5 minutes.
Slice the ciabatta rolls in half and drizzle with a little olive oil. Toast them on a griddle or under a grill until golden and crisp.
Thinly slice the rested steak. Spread 1 tbsp of craft ale mustard on the bottom half of each ciabatta roll. Layer with rocket, sliced steak, caramelised onions, and gruyere cheese. Top with the other half of the roll.
Serve the steak sandwiches immediately, accompanied by a side salad or crisps if desired, ensuring each bite is a perfect blend of tender steak, sweet onions, and tangy mustard.
