Preheat a skillet over medium-high heat.
Brush the sirloin steak with olive oil and season both sides with salt and black pepper.
Place the steak in the hot skillet and cook for about 4 minutes on each side for medium-rare, or to your preferred doneness.
Remove the steak from the skillet and let it rest on a plate covered loosely with foil.
In the same skillet, add the unsalted butter and sliced onions. Cook over medium heat, stirring occasionally, until the onions are golden brown and caramelized, about 8 minutes. Remove from skillet and set aside.
Reduce the heat to medium-low. Add the beef broth to the skillet, scraping up any browned bits from the bottom.
Stir in the heavy cream, green peppercorns, and Dijon mustard. Simmer gently for 3-4 minutes until the sauce thickens slightly. Adjust seasoning with salt and pepper as needed.
While the sauce simmers, heat a separate small skillet with vegetable oil over medium heat.
Toast the ciabatta rolls cut side down until golden brown, about 2 minutes.
Thinly slice the rested steak against the grain.
To assemble each sandwich, layer arugula on the bottom half of the toasted roll, followed by the sliced steak, caramelized onions, and spoon some peppercorn sauce over the top.
Cover with the top half of the roll and serve immediately.