Preheat the oven to 425 degrees F. Season the entire tenderloin with salt and pepper. Spread the duck rillette over the inside of the tenderloin, leaving about 1/2-inch around the edges. Gently roll the tenderloin up, like a jelly roll. Using butchers twine, make several knots, about 2 inches apart. This will secure the stuffing and aid in overall cooking. In a large saute pan, heat the olive oil. Sear the tenderloin for 2 minutes on each side. Remove from the pan and set aide. After the fillet has cooled, rub the entire tenderloin with the mustard. Roll the entire tenderloin in the cracked black pepper, crusting the entire tenderloin. Line a baking sheet pan with parchment paper. Lay the tenderloin on the pan and roast for about 30 to 35 minutes for medium rare. Remove from the oven and rest for 5 minutes before slicing. With a carving knife, slice the tenderloin into fillets, about 2 inches thick.