Caramelise the onions

• Melt the butter in a pan over medium heat.
• Add the sliced onion with a pinch of salt and the sugar.
• Cook until golden, soft and sweet, then remove and set aside.

Make the Big Mac sauce

• Mix the mayonnaise, yellow mustard, ketchup, white vinegar, sugar, salt and black pepper in a bowl.
• Stir in a little of the caramelised onions and set the sauce aside.

Smash the beef onto the tortillas

• Take a handful of ground beef and press it into a thin, even layer on one side of each tortilla.
• Season with salt, black pepper, all purpose seasoning and paprika.

Cook the smash tacos

• Heat a pan over medium-high heat and place a tortilla beef-side down, then smash firmly.
• Cook until browned and crispy, then flip and lay a cheese slice on the beef.
• Let the underside toast lightly until the cheese melts.

Assemble and serve

• Spread the Big Mac sauce over the melted cheese.
• Add caramelised onions and shredded lettuce.
• Fold or serve flat like a taco.