Make-Ahead: Combine first 3 ingredients. Cover; chill.
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 Serve: 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. Let stand 10 minutes before slicing. Serve with sour cream mixture. Garnish, if desired.