Rare: 4 minutes on each side Med-rare: 5 minutes on each side (recommended) Medium: 6 minutes on each side Well-done: Not recommended Substitute rump steak with sirloin or fillet steak
Start by making the sticky caramelised onions. Heat the butter, olive oil and sugar in a saucepan over a medium heat.
Add the onions and a big pinch of salt and cook slowly for about 5 minutes, stirring occasionally to prevent them from catching on the bottom of the pan.
When the onions are soft, stir in the red wine vinegar, cover with a lid and cook on a low heat for 25 minutes until sticky and caramelised.
In the meantime, rub the rump with olive oil, salt and freshly ground black pepper.
Grill the steak in a smoking hot griddle pan, turning to cook on both sides to your preferred doneness (see cooking guideline above).
Transfer the meat to a cutting board and allow it to rest for half the cooking time before slicing it into slivers.
To assemble the paninis, slice the ciabatta bread and generously spread hot English mustard onto the top and bottom slices. Top with the slow cooked sticky onions, juicy rump steak and fresh watercress.