Preheat a skillet or grill pan over medium-high heat.
Brush the sirloin steak with olive oil and season both sides with salt and freshly ground black pepper.
Place the steak on the hot skillet and cook for about 4 minutes on each side for medium-rare, or until desired doneness. Remove from heat and let rest for 5 minutes.
While the steak rests, melt the butter in a separate skillet over medium heat.
Add the sliced onions and cook, stirring occasionally, until caramelized and soft, about 8 minutes. Remove from heat and set aside.
Slice the rested steak thinly against the grain.
In the same skillet used for the steak, add the beef broth and bring to a simmer over medium heat.
Add the crushed green peppercorns and Dijon mustard, stirring to combine.
Pour in the heavy cream and continue to simmer gently until the sauce thickens slightly, about 3-4 minutes.
Season the peppercorn sauce with salt and black pepper to taste, then remove from heat.
Slice the ciabatta rolls in half and toast them lightly until golden.
To assemble each sandwich, layer arugula on the bottom half of the roll, then add sliced steak, caramelized onions, and spoon over the peppercorn sauce.
Top with the other half of the roll and serve immediately.