Mix the broth, onions, carrots, raisins, vinegar, orange zest, mushrooms, sage, and allspice in a 1-gallon plastic freezer bag. Seal tightly and freeze flat for up to 4 months.
To cook, remove the frozen braise base from the bag, then set in a 5- to 6-quart slow cooker, chipping up any bits so that the frozen base essentially sits flat.
Arrange the short ribs pieces and the bay leaf on top of the base. Cover and cook on low for 10 hours. The braise can stay on the keep-warm setting for up to 3 hours. Discard the bay leaf before serving.