Heat oil in a 3-quart heavy saucepan over moderately high heat until hot but not smoking, then reduce heat to moderate and cook onion and garlic, stirring, until onion is softened, about 2 minutes. Add Port, white wine, and bay leaf and boil until liquid is reduced to about 2 tablespoons, about 8 minutes. Add chicken stock and boil until liquid is reduced to about 1 cup, about 18 minutes. Add demiglace and boil, stirring occasionally, until liquid is reduced to about 1 cup again, about 10 minutes.
Pour sauce through a fine sieve into another saucepan, pressing on and discarding solids. Keep sauce warm, covered.
