Thinly slice the yellow onion into half-moons.
Heat half of the olive oil in a small skillet over medium-low heat, add the onions, and cook until soft and golden brown, about 15 minutes.
While onions cook, season the sirloin steak on both sides with sea salt, black pepper, and garlic powder.
Heat the remaining olive oil in a separate cast-iron skillet over medium-high heat.
Sear the steak for 3-4 minutes per side for medium-rare, then remove from the pan and let it rest for 5 minutes.
In a small bowl, whisk together the Greek yogurt and prepared horseradish to create the sauce.
Toast the sourdough slice until it is crisp and golden.
Thinly slice the rested steak against the grain.
Assemble the sandwich by spreading the horseradish sauce on the toast, followed by a layer of arugula, the sliced steak, and the caramelized onions.