Mix all of the big mac sauce ingredients with a whisk and set aside.
Take the minced beef, mould into a ball and press out into burgers.
Slice the burger buns to leave a top, a bottom and a middle piece and toast until golden brown.
Fry the burgers until just cooked.
Assemble the Big Mac by placing the sauce on the bottom bun, topped with diced onion, lettuce, 1 patty, a slice of cheese and diced gherkin. Place the middle bun on top of the patty, repeat the process and top with the bun.