Preheat a heavy skillet or griddle over high heat.

Divide the ground beef into 8 equal portions and form into loose balls. Season with salt and black pepper.

Place balls of beef on the skillet and smash them flat into thin patties using a spatula. Cook until the edges are crispy and the center is browned, about 2 minutes per side.

Add a slice of cheddar cheese on top of each patty during the last 30 seconds of cooking to melt.

Toast hamburger buns on the skillet until golden brown.

Mix the mayonnaise with truffle oil in a small bowl to make the truffle mayo.

Build the burger by placing a lettuce leaf on the bottom bun, followed by a beef patty, a generous spread of truffle mayo, and a few slices of red onion. Top with another patty and more truffle mayo before closing with the top bun.

For the fries, cut potatoes into thin strips and soak in water for 30 minutes to remove excess starch.

Heat vegetable oil in a deep fryer or large pot to 180°C.

Fry potatoes until golden and crispy, about 7-10 minutes. Drain on paper towels and season with salt.

Serve the smashed burger with a side of fries and a bowl of truffle mayo for dipping.