In a bowl, add mayonnaise, yellow mustard, tomato ketchup and finely chopped gherkins, Mix well and set aside.
Divide the beef mince into 2 portions (each 90g) and form 2 large meatballs.
Preheat your oven on the lowest available setting. Heat a dry frying pan over a medium-high heat. Place the burger buns face down onto the dry pan and toast until golden. Set aside.
Place each of the formed meatballs onto the hot pan. Using a burger press or spatula, smash the meatballs down to form burger patties slightly larger than the size of your burger buns. Season with sea salt & pepper. Reduce the heat to medium. Cook the burger patties for around 2 minutes on the first side, flip and cook for a further 1 1/2 minutes.
Assemble your burger: Dress the bottom and middle buns with the prepared special sauce. Top both with finely chopped onion and shredded iceberg lettuce. On the bottom bun, add the processed cheese slice. On the middle bun, add sliced gherkins.
Carefully place your cooked burger patties on to the dressed buns. Place the middle layer on top of the bottom layer. Top with the sesame seed bun top, wrap the burger loosely in foil and place in the oven at the lowest available setting for 2 minutes to warm through. Serve with french fries.