Thinly slice the yellow onion into half-moons.
Heat ghee in a small skillet over medium-low heat, add the onions, and cook until they are soft and golden brown, approximately 15 minutes.
Deglaze the onion pan with balsamic vinegar, stirring to scrape up any browned bits, then remove from heat and set aside.
Season the flank steak evenly on both sides with the sea salt and black pepper.
Heat avocado 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 and let it rest for 5 minutes.
Slice the rested steak thinly against the grain to ensure maximum tenderness.
Toast the sprouted grain bread slices until they are golden and crisp.
Assemble the sandwich by layering the fresh arugula, sliced steak, and warm caramelized onions between the toasted bread slices.