Thinly slice the yellow onion into half-moons.
Heat olive oil in a skillet over medium-low heat; add onions and a pinch of salt, cooking until golden and jammy, about 15 minutes.
Stir in balsamic vinegar to deglaze the pan, then remove onions from the skillet and set aside.
Season the steak with sea salt and black pepper on both sides.
Wipe the skillet clean, increase heat to medium-high, and melt the ghee.
Sear the steak for 3-4 minutes per side for medium-rare, then let it rest for 5 minutes before slicing thinly against the grain.
Slice the ciabatta roll in half and toast until the edges are lightly crisp.
Spread Dijon mustard on the bottom half of the roll, then layer with arugula, steak slices, and caramelized onions before closing the sandwich.