Caramelise the Onions: In a large, heavy-bottomed frying pan, melt 1 tbsp of the butter with the olive oil over a medium-low heat. Add the sliced onions and a pinch of salt. Cook slowly, stirring occasionally, for 20-25 minutes, until the onions are deeply golden, soft, and sweet. Don't rush this!

Deglaze the Pan: Once the onions are caramelised, stir in the brown sugar and balsamic vinegar. Cook for another 1-2 minutes until the vinegar has reduced and the onions are glossy and jammy. Scrape them out of the pan into a bowl and set aside.

Prepare the Patties: While the onions are cooking, place the beef mince in a bowl. Add the Worcestershire sauce, garlic powder, onion powder, and a generous amount of salt and pepper. Mix gently with your hands until just combined—be careful not to overwork the meat.

Shape the Patties: Divide the mince mixture in half. Shape each half into a thin, rectangular patty that is roughly the same size as your slices of bread. This ensures you get beef in every single bite.

Cook the Patties: Wipe out the frying pan you used for the onions. Place it over a medium-high heat. Cook the patties for 3-4 minutes per side, or until they are nicely browned and cooked through. I find that pressing them down lightly with a spatula helps create a great crust. Once cooked, transfer them to a plate to rest for a moment.

Assemble the Patty Melts: Now for the fun part. Lay out your four slices of bread. Butter one side of each slice generously—this will be the outside of your sandwich. On the un-buttered side of two slices, place a slice of Swiss cheese, followed by a beef patty, and then a generous pile of the caramelised onions. Top with the second slice of cheese.

Grill the Sandwiches: Place the remaining two slices of bread on top, butter-side up. Carefully place the assembled sandwiches into the clean pan over a medium heat. Cook for 3-5 minutes per side, until the bread is a deep golden brown, crisp, and the cheese is completely melted and oozing. You can press down gently with a spatula to help it along.

Serve Immediately: Slice the patty melts in half diagonally and serve straight away while they're hot, crispy, and gooey.