Slice the yellow onion into very thin half-moons.
In a small skillet over medium-low heat, melt the ghee. Add the onions and cook slowly for 15-20 minutes, stirring occasionally.
Once the onions are soft and golden, stir in the balsamic vinegar and cook for 1 more minute, then set aside.
Pat the steak dry with a paper towel and season both sides with sea salt and black pepper.
Heat the avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Place the steak in the hot skillet and sear for 3-4 minutes per side for medium-rare, or until it reaches your preferred level of doneness.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes before slicing thinly against the grain.
While the steak rests, toast the sourdough bread until golden and crisp.
Spread the Dijon mustard onto the toasted sourdough, then layer with the arugula, sliced steak, and the warm caramelized onions.