Heat the olive oil in a large non-stick frying pan and add the onions and paprika. Cook for 15 minutes until tender. Stir in the mustard, brown sugar and balsamic, then turn the heat up. Add 100ml water and cook for another 15 minutes until reduced and sticky.
Meanwhile, cook the burgers according to the instructions on the packet. Serve on the burger baps with the sliced tomatoes, rocket and a spoonful of the sticky onions on top.