1. Place the Chateaubriand on a cutting board long ends on the sides. Begin to lightly slice into the beef long ways turning the beef a quarter turn, and then slice a little more. Keep doing this until you have a uniformly flat piece of beef about ½ inch think. Season the meat with salt and pepper 2. Sauté the spinach in 2 tbsp of butter just until wilted set aside. 3. Mix together the remaining butter, fontina, hazelnuts, cream cheese, lemon juice, sage, and cooled spinach. Taste mixture and adjust seasoning with salt and pepper. 4. Spread mixture on rolled out Chateaubriand . Roll the beef back up, and truss. Season the outside with salt and pepper. 5. Roast in a 400 degree oven until desired doneness is achieved. About 30 minutes for medium rare or 130 degrees internal temperature. 6. Pull out of the oven and let rest for 5-10 minutes before slicing, and serving.