To make the patties, Mix the beef and seasoning together in a bowl. Split the mixture into two and shape into balls. Then flatten with the palm of your hand until 3cm (1-inch) thick. Place on a plate, cover with cling film and refrigerate for 30 minutes before cooking. Season the outsides just before cooking.
To make the truffle mayo, place all the ingredients, apart from the two oils and salt, in the bowl of a food processor or blender and blend together.
Combine the truffle and veg oil in a jug and drizzle the mixture very slowly into the mixture as the food processor is running. If you rush this step, you risk splitting the mayonnaise.
You’re looking for a thick and creamy texture. If it’s too stiff, add a few teaspoons of water to loosen. Add salt, more lemon juice and/or truffle paste according to your taste.
Keep in an airtight container in the refrigerator for up to 5 days.
To make the caramelized onions, add the oil to a wide-based pan over a medium–high heat. Add the onions, then turn the heat down to low and cook, stirring regularly, for about 30 minutes, until brown and caramelised. Remove the pan from the heat.
To make the burgers, add a little oil to a heavy-bottomed frying pan and heat over a high heat until very hot. Add the patties to the pan and cook for around 5–6 minutes on each side for medium and 8-9 minutes on each side for well done.
While they’re cooking, slice your buns in half and give them a good brush of melted butter on the cut side. Add them, cut side down, to a second frying pan over a medium heat for 3-5 minutes to toast them.
Once you’ve cooked the burgers on both sides, add the brie on top of them. Add a splash of water to the pan and then quickly cover with a lid so the steam melts the cheese.
When the buns are ready, remove them from the heat and spread the bottom buns with bacon jam. Then add a generous dollop of truffle mayonnaise to the top and bottoms of each bun.
When the cheese has melted and the burgers are cooked, get them out of the pan and place one on the base of each bun.
Return the pan to a medium heat; leave any remaining fat and juice from the burgers, as this will all add flavour. Add the caramelised onions and gently heat for 2-3 minutes while the burgers rest.
Add the caramelised onions to the top of the burgers and finish with the top of the buns.