Open the wine. Pour out 1/2 cup to enjoy while you carry on with the recipe. (This is not greed; a bit has to come out or the bottle with overflow when you add the other ingredients.)
Shave the zest from the orange with a vegetable peeler. Remove every trace of bitter white pith from the back with a sharp knife. Poke the orange zest into the bottle of wine. Funnel in the sugar. Drop in the coriander seeds, if using. Recork the bottle. Turn it upside down and right side up a few times until the sugar has dissolved. Refrigerate for 1 week, giving the wine a shake once a day.
Strain the wine into a carafe and serve well-chilled as an aperitif.