Heat the oil in a very large skillet over medium-high heat. Season the beef tenderloin with salt and pepper and cook to the desired level of doneness. Transfer to a serving plate and cover with aluminum foil. In the same skillet, melt the butter and sauté the shallots until they are tender. Add the water, port wine, sauce mix, and rosemary. Bring to a boil, stirring constantly. Reduce the heat and simmer for 2 or 3 minutes, stirring occasionally. Remove the rosemary sprig. Serve the sauce over the beef tenderloin.