Bring the steaks to room temperature about 30 mins before cooking. Brush with the oil and season with sea salt and freshly ground black pepper.
Heat a grill pan to medium-high and grill the steaks for about 2-3 mins per side for medium rare. Allow the steaks to rest for about 10 mins, then thinly slice them and set aside.
Cut the ciabatta rolls in half and grill for about 4-5 mins, taking care not to take on too much colour too quickly. Allow to cool and set aside.
Make the dressing for the coleslaw by combining the mayonnaise, yoghurt, mustard, vinegar, sugar, salt, pepper, stilton, celery salt and celery seeds, if using, in a small bowl. Mix and adjust the seasoning if necessary.
Mix together the cabbage, kale, carrots and spring onions in a bowl. Add the dressing and combine. Taste and adjust the seasoning, think salt, pepper and a pinch of sugar. Keep the coleslaw in an airtight container in the fridge until you’re ready to use it.
Place a small mound of rocket on the bottom half of the roll. Add the sliced steak and top with a large spoonful of coleslaw and the other half of the bun. Avoid adding too much juice from the steak or coleslaw to stop the buns going soggy.
If you're taking the sandwiches to a picnic, it's best to assemble them just before you leave home, or on-site if you like. Wrap the rolls in baking paper to transport them.