Caramelise the onions: Melt 1 tablespoon butter with the olive oil in a large frying pan over medium-low heat. Add the sliced onions, season with a pinch of salt, and stir to coat. Cook for 20-25 minutes, stirring every 3-4 minutes, until the onions are deep golden-brown and smell sweetly rich — they should look like soft, glossy ribbons, not burnt or crispy.

Prepare the patties: While the onions cook, divide the beef mince into 4 equal portions. Roll each into a ball, then flatten into thin patties about 1cm thick and slightly larger than your bread slices (they'll shrink a little). Season both sides generously with salt and pepper, and rub in the Worcestershire sauce.

Cook the patties: Once the onions are done, transfer them to a bowl and wipe the pan clean. Increase heat to medium-high and add the patties. Cook for 3 minutes on the first side — you'll see the edges turning brown and hear a satisfying sizzle. Flip and cook for another 2 minutes for medium. The patties should feel firm but have a little give when pressed. Remove and let rest on a plate.

Assemble the melts: Wipe the pan clean again and reduce heat to medium. Spread a thin layer of mayonnaise on one side of each bread slice (this replaces butter and gives a beautiful golden crust). Place 4 slices, mayo-side down, on a clean surface. Top each with a slice of cheese, one patty, a generous heap of caramelised onions, another slice of cheese, and finally a second bread slice with mayo-side facing up.

Toast the sandwiches: Place two sandwiches in the pan (don't overcrowd). Cook for 3-4 minutes per side, pressing down gently with a spatula after flipping. The bread should be deep golden-brown and the cheese visibly melted at the edges. You'll hear a gentle crackle when you press the spatula down — that's the sound of perfection.

Rest and serve: Transfer the finished melts to a cutting board and let rest for 1-2 minutes. This allows the cheese to set slightly so it doesn't all ooze out when you cut. Slice diagonally and serve immediately while the bread is still crisp.