1. Mix sauce and beef stock; heat to boil. Reduce heat to low simmer, and pull halfway off burner. Let reduce by half, removing skin from top each half hour or so. Finish to taste with wine, salt, white pepper.
  2. Note: As espagnole is composed of double strength beef stock a demi-glace is therefore about 2.5x strength. The flavor should be rich, deep and intense. A canned beef broth is not a good idea for demi-glace, as the "tin" taste will only intensify during the reductions involved.