Thinly slice the yellow onion into half-moons.
Heat half of the olive oil in a skillet over medium heat, add the onions, and cook until golden brown and sweet, approximately 10 to 12 minutes.
Deglaze the onions with the balsamic vinegar, stirring to incorporate, then remove from the pan and set aside.
Season the sirloin steak on both sides with the sea salt and black pepper.
Increase the skillet heat to medium-high, add the remaining oil, and sear the steak for 3 to 4 minutes per side until it reaches your desired level of doneness.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing against the grain into thin strips.
Toast the sourdough bread until it is golden and crisp.
Spread the Dijon mustard onto the toast, then layer with the arugula, the sliced steak, and the warm caramelized onions.