Thinly slice the yellow onion into half-moons.
Heat olive oil in a non-stick skillet over medium-low heat; add the onions and a pinch of sea salt.
Cook the onions for 12-15 minutes, stirring occasionally, until they are golden brown and soft; stir in the balsamic vinegar during the last minute of cooking.
While onions cook, season both sides of the sirloin steak with the remaining sea salt and black pepper.
Remove onions from the pan and set aside. Increase the heat to medium-high.
Place the steak in the hot skillet and sear for 3-4 minutes per side for medium-rare, or until it reaches your preferred 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 fresh arugula, the sliced steak, and the caramelized onions before closing the sandwich.