Caramelised Onions: Add butter and olive oil to a pan over medium heat, then add in the onions. Fry for 15mins, stirring somewhat frequently, until the onions turn soft and golden. Stir in balsamic vinegar, sugar and salt & pepper, then lower the heat slightly. Continue frying for another 15-20mins, or until completely soft, sticky and sweet. If any fond (flavour/char) builds up in the pan at any point, just add a dash of water and use your wooden spoon to scrape it off. The lower and slower you fry the softer and sweet the onions will be. Remove onions from pan when done.
Prep Steak: Pat both sides of the steaks dry then generously season all over with salt & pepper.
Cook Steak: Place a large heavy-based/cast-iron pan over high heat. Once the pan is smoking hot (literally) add a drizzle of oil. Leave for 10 seconds, then add in the steaks. Fry for 2mins (should see blood/moisture rise to the surface). Flip then fry for 1 minute. Add in the butter, rosemary, garlic & thyme and baste the steaks for a further minute. Remove steaks from pan & leave to rest (important) and thinly slice just before needed. Leave herbs and garlic in the pan.
Sauce: Lower heat to medium and pour in the stock and cream. Scrape off any flavour from the pan, then stir in blue cheese and chives. Simmer for around 5 mins, stirring somewhat frequently, or until the cheese has melted and the sauce has thickened. Pour in the resting juices from the steak and remove the garlic/rosemary/thyme. Simmer for a minute or so until it re-thickens. It will thicken, just keep simmering/stirring (it will thicken more as it rests).
Bread: Whilst it's simmering halve the bread and place cut-side-up on a baking tray. Brush with 1 tbsp oil then place under the grill until golden and crisp.
Sandwich: Top the bottom slice of ciabatta with caramelised onions, followed by tomatoes with a pinch of salt and pepper. Top with rocket, then steak, then blue cheese sauce (measured with love). Pop the lid on, slice in half and enjoy!