For the creamy herb sauce: In a small bowl, combine the mayonnaise, sour cream, rosemary and mustard and mix well. Cover and chill for up to 3 days.
For the roast: Preheat the oven to 250 degrees F. Line a roasting pan with aluminum foil. Pour the beef broth into the roasting pan. Place the rack over the broth, and spray with nonstick cooking spray.
In a small bowl, combine the rosemary, salt and pepper. Rub the mixture evenly over the roast.
In a large skillet, heat the olive oil over medium-high heat. Add the roast and cook until browned, 2 to 3 minutes per side. Place the roast on the prepared rack in the roasting pan. Cover tightly with aluminum foil. Bake for 45 minutes to 1 1/2 hours (depending on the shape of the roast), or until a meat thermometer inserted in the center of the roast reads 125 degrees F.
Remove from the oven and let stand for 20 minutes before slicing. Serve with the creamy herb sauce and garnish with rosemary if desired.