Cheeseburger with caramelized onions and mustard mayo

Break up the ground beef in a bowl and add an egg to bind.

Shape into balls and press flat into round patties. Chill in the fridge as long as possible.

Prep all toppings before cooking: slice onion into big pieces; mix mayo with Dijon mustard.

Season the patties heavily with salt and pepper on both sides and the edges; coat with olive oil.

Caramelize the onions in a little olive oil.

Cook the burgers, flip, add butter on top and baste. Season again with salt and pepper.

Butter the buns and grill both cut sides. Add cheddar to the burgers.

Assemble: mustard mayo on the bun, lettuce shaped to the bun, tomato, more salt and pepper, more mustard mayo, the burger, onions, top bun.