1. Mix all the ingredients for the burger sauce and put aside.
2. In a large bowl, pull the meat apart into small chunks, add salt and pepper and toss gently with fingers spread apart until loosely mixed.
3. Using wet hands, shape the mixture into 4 burgers.
4. Heat a griddle pan to hot. Cook for 3-4 minutes each side or until well browned and cooked.
5. Top each burger with a slice of cheese and leave to rest aprox. 3-4 min.
6. Toast the burger buns until lightly golden.
7. Spread the burger sauce over the base of each bun. Add the burger and then some tomato, lettuce, cucumber, red onions and cornichons. Top with the other half of the bun.
8. Serve with a small bowl of ketchup and fries, if you like.