Thinly slice the yellow onion into half-moons.
Heat 1 tsp of olive oil in a small skillet over medium-low heat, add the onions and a pinch of salt, and cook until golden brown and soft, about 10-12 minutes.
Stir the balsamic vinegar into the onions during the last minute of cooking to glaze them, 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 and add the remaining 1 tsp of olive oil.
Sear the steak for 3-4 minutes per side for medium-rare, or until it reaches your desired level of doneness.
Remove the steak from the pan and let it rest on a cutting board for 5 minutes before slicing thinly against the grain.
Lightly toast the sprouted grain bread slices until they are golden and crisp.
Spread the Dijon mustard onto one slice of toast, then layer with the arugula, sliced steak, and balsamic caramelized onions.
Top with the remaining slice of bread and slice the sandwich in half to serve.