Put the butter and oil in a large non-stick frying pan. Add the onion and fry over a medium heat for 15 mins or until golden and sticky. Stir through the light brown soft sugar, cider vinegar and wholegrain mustard. Heat the grill to medium-high. Put the beef burgers on a baking sheet and grill on each side for 5-8 mins or until cooked through. For the last minute, top the burgers with the gruyère and grill until bubbling and melted. Lightly toast the brioche buns. Top the buns with the burgers, then the sticky onions and divide the watercress between each.