Thinly slice the yellow onion into half-moons.
Heat the olive oil in a small skillet over medium-low heat, add the onions, and cook until soft and golden brown, approximately 12-15 minutes.
Stir the balsamic vinegar into the onions and cook for 1 additional minute until glazed, then remove from the pan and set aside.
Season both sides of the sirloin steak with sea salt and black pepper.
Increase the skillet heat to medium-high and sear the steak 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.
Toast the sourdough bread until golden and crisp, then lightly rub the surface with the raw garlic clove.
Spread the Dijon mustard onto the toasted bread, then layer with baby arugula, the sliced steak, and the caramelized onions.