Start by making the caramelised onions. Heat the butter and oil in a saucepan over a medium heat. Add the onions and a big pinch of salt and cook slowly for about 10 minutes, stirring occasionally to prevent them from catching on the bottom of the pan.
Whilst the onions are cooking, make the aioli. Mix ingredients together until well combined and set aside until serving to allow the flavour to develop.
When the onions are soft and turning golden brown, stir in the sugar and balsamic vinegar and cook on a low heat for a further 5 to 10 minutes until sticky and caramelised.
Prepare the rump by rubbing the steak with olive oil and seasoning well with salt and freshly ground black pepper.
Grill the steak in a smoking hot griddle pan with olive oil, turning to cook on both sides to your preferred doneness (see cooking guideline above). Leave the meat to rest for half its cooking time before slicing it into slivers.
Generously spread the garlic aioli onto the bottom slice of bread and top with lettuce, tomato, caramelised onions, juicy steak slices and finally some mustard on the top slice of rosemary bread. Serve with extra aioli.