Sauté the Onion: Heat the olive oil in a medium saucepan or large, deep frying pan over a medium heat. Add the sliced onion and cook gently for 8-10 minutes, stirring occasionally, until it's very soft, sweet, and lightly caramelised.

Add Aromatics: Add the minced garlic and cook for another minute until fragrant. Be careful not to let it burn.

Create the Base: Sprinkle the plain flour over the onions and stir continuously for one minute. This will cook out the raw flour taste and help to thicken the jus slightly.

Build the Jus: Gradually pour in the beef stock, whisking as you go to prevent any lumps from forming. Once all the stock is incorporated, add the Worcestershire sauce, soy sauce, thyme, and the bay leaf.

Simmer for Flavour: Bring the mixture to a gentle boil, then reduce the heat to low and let it simmer for at least 20 minutes. This allows the flavours to meld and the sauce to reduce and concentrate. I find that this is the most important step for a truly rich jus.

Season and Strain: Taste the jus and season with salt and pepper as needed. For a smooth dipping sauce, you can strain the jus through a fine-mesh sieve, pressing on the onions to extract all the liquid. Discard the solids. If you prefer a more rustic texture, you can leave the onions in.

Warm the Beef: Return the jus to the pan and bring it back to a gentle simmer. Add the sliced roast beef to the jus and let it warm through for just 1-2 minutes. The goal is just to heat the beef, not to cook it further.

Prepare the Rolls: While the beef is warming, slice your rolls horizontally, but not all the way through. Spread the softened butter on the cut sides and place them under a hot grill for 1-2 minutes, or until golden and toasted.

Assemble the Sandwiches: Using tongs, lift the warm beef from the jus, allowing any excess to drip off, and pile it generously onto the bottom half of each toasted roll. If using cheese, lay two slices over the hot beef and pop back under the grill for a minute until the cheese is gloriously melty and bubbling.

Serve Immediately: Place the top on each sandwich, cut in half if you like, and serve immediately. Ladle the hot jus into individual small bowls for dipping.