Remove the New York strip steak from the fridge at least 1 hour before cooking and season both sides with salt and black pepper.

Heat a non-stick skillet over medium-high heat and sear the steak for 1 minute on each side until golden brown, basting with butter if desired. Let it rest for 5 minutes before slicing thinly.

In the same skillet, add the sliced onions and salt, cooking over medium heat for 10-15 minutes until caramelized, then stir in the balsamic glaze.

Slice the ciabatta in half and toast until golden and crispy.

Assemble the sandwich by spreading aioli on the toasted ciabatta, layering with arugula, sliced steak, and caramelized onions.

Cut the sandwich in half and serve warm.