Thinly slice the yellow onion into half-moons.
Heat 0.5 tsp avocado oil in a non-stick skillet over medium-low heat; add onions and cook until deeply golden and sweet, about 15 minutes.
Season both sides of the sirloin steak with sea salt and black pepper.
Remove onions from the pan, increase heat to medium-high, and add the remaining oil.
Sear the steak for 3-4 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
Slice the rested steak thinly against the grain to ensure maximum tenderness.
Toast the sourdough slices until golden, then spread Dijon mustard on one side.
Assemble the sandwich by layering the arugula, steak slices, and caramelized onions between the bread.