2) While the onions and apples are cooking, blend together in a large bowl the beef mince, salt, pepper, thyme, Dijon, egg, and breadcrumbs. Be sure the ingredients are well incorporated, but don't over work. Form the mixture into 4 good-sized burgers.
3) Heat the griddle to medium-high heat. Place the burgers on the griddle and cook about 10 minutes per side, or until cooked through. As the burgers are finishing (after the first flip), spread mayonnaise on both cut sides of each split roll. Place the rolls on the griddle, mayonnaise side down, to toast for about 1 minute, or until golden.
4) Place each burger on the bottom half of a toasted roll. Top with about 1/4 of blue cheese, then with a generous scoop of the caramelised onion and apple mixture. Cover with the roll top and serve.
Cook's Tip: If you create a concave burger, with the centre thinner than the edges, when it cooks the burger won't swell up to look like a meatball, but will remain flat and patty-like.