With all the holiday entertaining, it’s nice to have a couple of good appetizers up your sleeve — something that can be served to guests or brought along to a party. Something that can liven up a meal of leftovers, or even make for lunch on the day when a festive dinner is taking up your attention. This chicken liver mousse is one of my absolute favorites. It can (should) be made ahead, is just decadent enough, and is cheap to make. I buy organic, air-chilled chicken livers at Whole Foods (because they always have them) and they cost $1.50 for the whole recipe’s worth. [...]