Melt the butter in a frying pan over a medium heat and add the onions, thyme, sugar and a pinch of salt. Cook for ten minutes until the onions are soft and golden.
Add the ale and mustard and cook for a further five minutes.
Heat a little olive oil in another frying pan on high. Season the steak on both sides with salt and pepper then add to the hot pan.
Cook for five to six minutes on each side for medium/well.
Remove the steak and allow to rest for ten minutes.
Slice the ciabatta in half. Brush each half with a little olive oil and place cut side down in a hot grill pan just to char a little then remove.
Spoon the onions over one half then top with slices of the cheddar. Place under a hot grill for a few minutes to melt the cheese.
Place a handful of the rocket on top of the cheese followed by slices of the rested steak.
Top with the other half of the ciabatta then slice in half and serve.