WHEN WARM, add diced orange, the sugar and the spices. Stir a little and then leave on very low heat for a few minutes. Have a taste, take off heat when you are satisfied with the flavor (when the spices have had enough time to infuse into the wine). Pass the wine through a sieve to catch all the spices and orange cubes.
Serve the mulled wine in heat-proof glasses with a slice of orange on the side.