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 they are soft and golden brown, about 15 minutes.
Pour the balsamic vinegar into the skillet to deglaze the pan, stirring it into the onions until the liquid has mostly evaporated, then remove the onions from the pan.
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 olive oil.
Sear the steak for 3 to 4 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
Slice the steak thinly against the grain to ensure maximum tenderness.
Split the sprouted grain bun and toast it until the edges are lightly crisp.
Spread the Dijon mustard on the bottom bun, then layer with the baby arugula, sliced steak, and the caramelized onions before closing the sandwich.