For the garlic aioli mix together the mayonnaise and garlic, lemon juice, and salt & pepper. Set aside for 30 minutes or so to allow the flavours to develop.
For the caramelised onions melt the butter in the Chef Pan or Paella Pan over a medium heat. Add the onions and cook on a low heat with the lid on for 10-12 minutes or until starting to look golden and soft. Add the sugar, vinegar, salt and pepper and continue to cook for another 10 minutes more until it is very soft. Remove from heat and keep warm.
Season the steaks all over with salt, pepper and brush with oil. Heat the BBQ Grid to hot and add the steaks. Turn the heat to medium and cook for 3 minutes on each side for medium, or cook to your liking. Remove the steaks from the heat, cover with foil and let them rest for a few minutes before slicing, toss them in the resting juices with a drizzle of oil.
While the steak is cooking, slice the rolls or ciabatta in half and put cut side down on to the BBQ Grid to warm through slightly and soak up some of the flavour from the fried steaks. Spread each side generously with the garlic aioli. Divide the onions and rocket between the sandwich bottoms then top with the sliced steaks and sandwich tops, dig in immediately.