Season the steaks with sea salt and cracked pepper.
In a large frying pan, melt the butter over moderate / high heat.
Cook the steaks for about 30 to 40 seconds each side for medium
Set aside for a minute to rest.
Slice the croissants in half lengthwise. In 2 batches, place the croissant halves, cut side down in the frying pan and cook for 30 seconds to 1 minute or until lightly toasted.
Remove from pan and place on plates.
Lay the steak on four of the toasted croissant halves then top with lettuce and another croissant half.