Start the caramelised onions. Place a large frying pan over medium-low heat and add the butter and olive oil. Once the butter has melted and is gently sizzling, add the sliced onions. Stir to coat them in the fat. Cook for 20-25 minutes, stirring every 3-4 minutes. You’ll notice the onions soften and become translucent, then slowly turn a deep golden brown, releasing a sweet, nutty aroma. When they are a rich amber colour and have reduced significantly in volume, sprinkle over the caster sugar and balsamic vinegar. Cook for a further 2 minutes, until the liquid has evaporated and the onions look glossy and jammy. Remove from the heat and set aside.

Shape the patties. While the onions cook, place the beef mince in a large bowl. Season with the salt and pepper. Using your hands, gently combine the seasoning into the mince. Divide the mixture into 4 equal portions (about 125g each) and shape each into a patty about 2.5cm thick. Make a small indent in the centre of each patty with your thumb – this prevents them from doming up during cooking. You should feel the cool, soft texture of the raw beef, and the patties should be uniform in shape.

Preheat the grill or griddle. Heat your barbecue or a cast-iron griddle pan over high heat. You want the cooking surface to be smoking hot – hold your hand about 15cm above it; you should be able to hold it there for only 1-2 seconds. This high heat creates the essential sear.

Grill the patties. Place the patties on the hot grill. Listen for a loud, satisfying sizzle as they hit the metal. Cook for 3-4 minutes without moving them. You’ll see the edges turning brown and the juices beginning to bubble on the surface. Flip the patties carefully with a spatula. You should see deep, dark grill marks and a beautiful browned crust. Cook for a further 2 minutes, then place a slice of cheese on each patty. Cover the grill or tent loosely with foil for the final minute of cooking, until the cheese is melted and gooey.

Toast the buns. In the last minute of cooking, place the split buns cut-side down on the cooler edges of the grill. Toast for about 30-60 seconds, until they are lightly golden and have a crisp, warm surface. You’ll smell the toasted bread aroma.

Rest the patties. Remove the patties from the grill and let them rest on a clean plate for 2 minutes. This allows the juices to redistribute, ensuring a moist burger. You’ll see a small pool of juices forming on the plate – this is the flavour you want inside the patty.

Assemble the burgers. Place a lettuce leaf on the bottom half of each toasted bun. Top with the cheesy patty, a generous spoonful of caramelised onions, and a slice of tomato. Add gherkins if using, then crown with the top bun. Serve immediately while the cheese is still molten and the bun is warm.