Caramelise the Onions: Place a large frying pan over a low-medium heat. Add the 30g of butter and olive oil. Once the butter is melted, add the sliced onions and a pinch of salt. Cook slowly, stirring occasionally, for 25-30 minutes. They should be very soft, sweet, and a deep golden brown. Avoid high heat, as you want to coax the sugars out gently.

Finish the Onions: Once the onions are deeply caramelised, stir in the brown sugar and balsamic vinegar. Cook for another 2-3 minutes until the liquid has evaporated and the onions are jammy. Season with pepper, then remove from the pan and set aside.

Prepare the Patties: In a medium bowl, gently combine the beef mince, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. I find that using my hands is best, but be careful not to overwork the meat. Divide the mixture into four equal portions and shape them into thin, rectangular patties that will fit the shape of your bread slices.

Cook the Patties: Wipe out the same frying pan you used for the onions and place it over a medium-high heat. Cook the patties for 3-4 minutes per side, until they have a lovely brown crust and are cooked through. If you like a really crispy edge, you can gently press them down, similar to how we make our Smashed Burgers Recipe. Remove the patties from the pan and let them rest on a plate.

Assemble the Melts: Lay out your 8 slices of bread. Butter one side of each slice generously. On the un-buttered side of four slices, layer your ingredients. Start with a quarter of the mixed Cheddar and Gruyère cheeses, followed by a cooked beef patty, then a generous pile of the caramelised onions, and finally another quarter of the cheese. Top with the remaining four slices of bread, butter-side-up.

Grill the Patty Melts: Carefully clean the frying pan once more, or use a clean one, and place it over a medium heat. Place two of the assembled sandwiches into the pan and cook for 3-4 minutes per side. The bread should be golden brown and crisp, and the cheese should be completely melted and gooey. I find that placing a lid on the pan for the last minute helps the cheese melt beautifully. Repeat with the remaining two sandwiches.

Serve Immediately: Slice the patty melts in half diagonally and serve straight away while they are hot and the cheese is still wonderfully stringy.