Thinly slice the yellow onion into half-moons.
Heat half of the olive oil in a skillet over medium-low heat, add the onions, and cook for 15 minutes until soft and golden.
Stir the balsamic vinegar into the onions and cook for another 2 minutes until glazed, then remove from the pan.
Season the sirloin steak on both sides with sea salt and black pepper.
Increase the skillet heat to medium-high and add the remaining olive oil.
Sear the steak for 3 to 4 minutes per side for medium-rare, or until it reaches your preferred level of doneness.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing thinly against the grain.
Slice the ciabatta roll in half and toast until the edges are lightly crisp.
Spread the Dijon mustard on the bottom half of the roll, then layer with the sliced steak, caramelized onions, and fresh arugula before closing the sandwich.