Following package directions, add water to 3 oz. beef demi-glace concentrate to make 1 1/2 cups.
In a medium saucepan over medium heat, cook 1 tbsp. minced shallot with 2 tbsp. unsalted butter, stirring occasionally, until translucent, 5 minutes. Add demi-glace and 6 oz. black lager. Bring to a boil, whisking.
In a small bowl, whisk 1 tbsp. each cornstarch and cold water; whisk into lager mixture, return to simmering, and cook about 1 minute. Season to taste with kosher salt and freshly ground pepper.
MAKE AHEAD Chilled, up to 1 day; reheat, stirring.
*Find Classic French Demi-Glace at well-stocked grocery stores and at morethangourmet.com ($5.25/1.5 oz.).