Slice the yellow onion into thin half-moons.
Heat the ghee in a small skillet over medium-low heat; add onions and cook, stirring occasionally, until deeply golden and caramelized, about 15 minutes.
Stir in the balsamic vinegar to deglaze the pan, then remove from heat.
Season the sirloin steak on both sides with sea salt and black pepper.
Heat the olive oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the steak for 3 to 4 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
Toast the sprouted grain bread slices until golden brown.
Thinly slice the rested steak against the grain into strips.
Spread Dijon mustard on the toasted bread, then layer with the sliced steak, caramelized onions, and fresh arugula.