1) Put the beef in a bowl, add salt and pepper and mix with the truffle oil.
2) Make 4 circular hamburgers, weighing about 150g each.
3) Place all the ingredients for the mayonnaise in a blender and mix on a medium speed until it reaches the consistency of mayonnaise.
4) Place the onion in a frying pan and cook until soft, deglaze with a small glass of cognac.
5) Cook the burgers under the grill or in a grill pan. Once cooked to your liking, top with the cheddar cheese.
6) Cut the rolls and spread the black truffle pate inside, put the cheese-topped hamburger between the bun and finish by adding onion and mayonnaise on the top to serve.
