1 To make the onions, finely slice red onions then lightly pickle them using sugar, salt and vinegar.
2 Peel 1 potato and slice into matchsticks using a knife or mandolin. Soak in water to drain off the remaining starch, then pat dry right before frying.
3 To make the mustard mayo, mix mustard and mayonnaise together.
4 Season the steak and grill on both sides until cooked to your liking and leave to rest.
5 Heat oil to 170C and fry matchstick potato until crisp. Drain off excess oil and season with salt.
6 Slice into a large ciabatta and grill until toasted, generously spread mustard mayo on both sides, followed by lettuce, sliced steak, and pickled onions.
7 Gently pull apart the Dolcelatte cheese and liberally place on top of the sandwich. Grill or blow torch to lightly melt the cheese then top with the matchstick potato. Close the sandwich with the last slice of ciabatta and enjoy.