In a frying pan sauté the onions and the red peppers in a splash of oil until softened and starting to caramelise. add the vinegar and sugar and reduce until sticky and set aside.
(If you have no leftovers) Heat a splash of oil in a frying pan over a high heat. Season the steak with salt and sear it for 2-3 minutes each side for medium rare or 3-4 minutes for medium/ medium well. Allow to rest in a warm place.
Under a high grill slice the ciabattas and toast lightly. Mix together the mayonnaise and the mustard together spread this on the bottom slice top with the sliced beef, caramelised onions and cheddar cheese and return to the grill until melted.
Top with fresh rocket and the lid, best eaten straight away.