Place peppercorns in container of an electric blender; cover and pulse until chopped.
Transfer to a bowl, and stir in salt.
Place beef on a lightly greased rack in a shallow roasting pan.
Combine parsley, butter, and 3 tablespoons mustard; rub mixture evenly over tenderloin. Pat peppercorn mixture evenly over beef. Cover; chill up to 24 hours.
When Ready to
Bake at 350 for 50 minutes or until meat thermometer inserted in thickest portion of beef registers 145 (medium-rare) to 160 (medium).
Transfer beef to a platter; cover loosely with aluminum foil.