Scribbled Underline

– 1 ½ pound beef tenderloin – ½ cup stone ground dijon mustard – 1/4 cup freshly ground coarse black pepper – 2 tablespoons olive oil plus more for drizzling – 2 tablespoons salted butter – coarse sea salt to taste – ¼ cup capers drained – ½ cup baby arugula – ½ lemon or to taste