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 until soft and golden, about 15 minutes.
Stir the balsamic vinegar into the onions and cook for 1 additional minute before removing them from the skillet.
Season both sides of the sirloin steak with sea salt and black pepper.
Increase the skillet heat to medium-high, add the remaining olive oil, and sear the steak for 3 to 4 minutes per side for medium-rare.
Transfer the steak to a cutting board and let it rest for 5 minutes before slicing thinly against the grain.
Toast the sprouted grain bread slices until they are golden and crisp.
Spread the Dijon mustard onto one slice of toast, then layer with arugula, the sliced steak, and the caramelized onions.
Top with the remaining slice of bread and serve immediately.