Caramelise the red onions

• Melt the butter in a frying pan over medium-low heat and add the sliced red onions.
• Cook slowly until soft and golden, then stir in the brown sugar and balsamic vinegar.
• Cook until sticky and jammy, then set aside.

Make the garlic Parmesan sauce

• Mix the mayonnaise with the grated Parmesan, crushed garlic and lemon juice.
• Season with a little salt and pepper and stir until thick and smooth.

Shape and season the patties

• Divide the beef mince into four equal balls and flatten into patties slightly wider than the buns.
• Season both sides with salt and pepper.

Cook the patties

• Heat the olive oil in a frying pan over high heat.
• Sear the patties for a few minutes on each side until well browned.
• Lay a slice of American cheese on each patty and let it melt.

Toast the buns

• Toast the cut sides of the buns in the pan until golden.

Assemble the burgers

• Spread garlic Parmesan sauce on the bun bases.
• Stack two cheesy patties per burger and top with the caramelised red onions.
• Close the buns and serve with the remaining sauce for dipping.