Add wine, cider, honey, orange zest, and orange juice to a 3-quart or larger slow cooker. Stir to combine. Add the cloves, cardamom, cinnamon, and star anise. Cook on low until warm, 30 minutes to 1 hour, depending on your slow cooker. Stir in the brandy.
Ladle into mugs and serve with an orange slice and cinnamon sticks. To keep the wine warm, leave the slow cooker on the "Keep Warm" setting. If your slow cooker does not have a warm setting, alternate between "Low" and "Off."
This recipe scales easily. Feel free to increase the batch size by as much as your slow cooker or pot will hold.
TO MAKE ON THE STOVE: simmer the wine, apple cider, orange zest and juice, and spices in a large pot or Dutch oven the stove over low heat for 30 minutes, instead of heating in the slow cooker. Do not let the wine boil Add the brandy and serve.
TO STORE: Let your slow cooker mulled wine cool completely, then pour it into an airtight storage container and place in the refrigerator for up to 3 days.
TO REHEAT: Gently rewarm mulled wine in a large pot on the stovetop over low heat, or pour leftovers into your slow cooker and reheat on LOW until warm.