In a 1 1/2- to 2-quart pan over medium heat, melt 1 tablespoon butter; add 2 tablespoons minced shallots and stir often until limp but not brown, 2 to 3 minutes.
Add 1/2 cup brandy and 1 tablespoon sherry vinegar; increase heat to medium-high and boil, stirring often, until liquid is almost evaporated, 5 to 6 minutes.
Add 2 cups veal or beef demiglace (see notes); reduce heat and simmer, stirring occasionally and skimming off any residue that comes to the surface, for 10 minutes. Stir in 2 tablespoons drained green peppercorns in brine and salt to taste. Just before serving, over low heat, whisk in 2 more tablespoons butter; pour into a gravy boat.
