Pickles: Heat the vinegar, water, sugar and salt in a small saucepan until the sugar and salt have dissolved. Allow to cool and pour over the thinly chopped vegetables. Place in the fridge until serving.
Caramelised onions: Chop the onions into long strips and fry slowly in oil over a low heat, stirring from time to time (around 30 minutes).
Mayonnaise (hand blender method): Starting with the egg, put all the ingredients for the mayonnaise into the blender jug (except the seasoning, rosemary and chili). Attach to the blender, turn on and let it do its work. Lift off carefully. Your mayonnaise is done. Finely chop the rosemary and chili and stir in gently.
Burgers: Fry the mushrooms in a dry pan until all the liquid has evaporated. Add butter, onion and garlic (finely chopped). Fry until the onion is glazed. Place the mushrooms in a food processor with all the other ingredients apart from the walnuts. Blend for roughly 10 seconds. Roast the walnuts in a dry pan and coarsely chop them. Form burgers with wet hands. Leave to rest for at least 15 minutes. Fry the burgers in plenty of oil and butter, or grill them on the barbecue.
Bread: Toast the bread in the oven topped with a Jarlsberg® cheese slice. Assemble your burger. Burger bun, caramelised onion, lettuce, burger, mayonnaise, pickles, bread and cheese.
Serve immediately and enjoy!
For this recipe you can use Jarlsberg® Original, Lite or Special Reserve.