Slice your clementines or oranges, split the vanilla bean, and lightly crush cardamom pods. This helps the flavors infuse the wine fully.
Pour the red wine into the slow cooker. Add all the spices, citrus slices, brandy, sugar, and any optional dried fruit.
Set the slow cooker to low and let the wine warm for about 2 hours. Avoid boiling, as it can make the wine taste bitter.
After 2 hours, taste the wine and adjust sweetness or spice levels if needed. Stir gently to mix everything evenly.
Ladle the mulled wine into heatproof mugs or glasses. Garnish with extra cinnamon sticks, citrus slices, or rosemary sprigs if desired.