Thinly slice the yellow onion into half-moons.
Heat 1 tsp of olive oil in a skillet over medium-low heat and add the onions with a pinch of salt.
Cook the onions slowly, stirring occasionally, until they are soft and deep golden brown, about 15 minutes.
Stir in the balsamic vinegar to deglaze the pan, then remove the onions and set them aside.
Season both sides of the sirloin steak with sea salt and black pepper.
Wipe the skillet clean, add the remaining 1 tsp of olive oil, and increase the heat to medium-high.
Sear the steak for 3 to 4 minutes per side for medium-rare, then transfer to a board to rest for 5 minutes.
Slice the steak thinly against the grain.
Split the sprouted grain roll and toast it until lightly crisp.
Spread Dijon mustard on the bottom roll, then layer with fresh arugula, steak slices, and the caramelized onions.