Wash and slice the orange, removing obvious seeds.
Slice the fresh ginger.
Pour the apple cider into a 5- to 6-quart slow cooker.
Add the orange slices.
Add cinnamon sticks, cloves, allspice berries, star anise, and ginger.
Stir in 2 tablespoons brown sugar.
Add fresh cranberries if using.
Add a small pinch of freshly grated nutmeg.
Stir gently.
Cover and cook on LOW for 3–4 hours.
After approximately 3 hours, carefully taste the cider.
Add additional brown sugar if necessary.
Remove the cooked fruit and whole spices, or strain the wassail through a fine-mesh strainer.
Return the wassail to the slow cooker if strained.
Switch the slow cooker to WARM for serving.
Ladle into heat-safe mugs.
Garnish with fresh orange slices and optional cinnamon sticks.
Serve warm.