Toast the brioche buns, then spread the mayonnaise and Dijon mustard on the bottom half and the tomato chutney/relish on the top half of each bun. Next, add a couple of leaves of lettuce, followed by a couple of slices of tomato and gherkin. Place the beef patties and cheese on top of the salad. Finally, add the caramelised onions on top of the patty, before placing the top half of the brioche bun on top. If the burgers are large, cutting them in half will help.