Prepare the croissant buns

• Slice the croissants in half horizontally.
• Gently flatten each half by pressing with your hand or a flat pan to create a level surface.
• Toast the cut sides in a pan with a little butter until golden and crisp, then set aside.

Make the special burger sauce

• Mix the mayonnaise, ketchup, yellow mustard, pickle juice, black pepper, sugar, diced pickles and salt together in a bowl.

Caramelize the onions

• Heat the butter and oil in a pan over medium heat.
• Add the sliced onion, sugar and salt and cook, stirring occasionally, until soft and golden.

Make the smash burgers

• Roll the beef into 2 loose balls.
• Heat a cast-iron skillet or heavy pan over medium-high heat until hot.
• Place a beef ball on the pan, cover with parchment paper and smash flat with a metal spatula or burger press, then season with salt and pepper.
• Cook until the edges are browned and crispy, then flip, add a slice of cheese, cover with a lid to melt and repeat for the second patty.

Assemble the burgers

• Spread the special sauce on the toasted croissant bottoms.
• Layer with lettuce, tomato and the cheesy burger patties.
• Add caramelized onions or pickles, then top with the croissant lid.