1. In a medium saucepan, combine 1 ¼ cups water with the demi-glace, beef base, chicken base, peppercorns, garlic powder, thyme, bay leaf, and white pepper. Whisk well.
2. Bring to a boil over medium-high heat and cook at a boil, uncovered, whisking occasionally, for about 25 minutes, or until glossy and smooth.
3. Strain through a chinois or fine-mesh sieve into a metal bowl. Discard the solids.
4. Let cook, then cover and refrigerate for at least 1 hour until chilled. Scrape off any fat that has congealed on the surface.
5. Use right away or transfer to a lidded container and refrigerate for up to 3 days.

Makes about 1 generous cup.