Thinly slice the yellow onion into half-moons.
Heat the olive oil in a skillet over medium-low heat and cook the onions, stirring occasionally, until they are soft and golden brown.
Season the sirloin steak on both sides with sea salt and black pepper.
Increase the skillet heat to medium-high and sear the steak for 3 to 4 minutes per side until it reaches your desired level of doneness.
Remove the steak from the pan and let it rest for 5 minutes before slicing thinly against the grain.
Toast the sourdough bread slices until they are golden and crisp.
In a small bowl, whisk together the Greek yogurt and horseradish until smooth.
Spread the yogurt mixture onto the toasted bread, then assemble the sandwich by layering the arugula, sliced steak, and caramelized onions.