Wash the oranges thoroughly in hot water, squeeze out the juice and reserve the leftover orange pieces.
Add frozen blueberries and the squeezed-out orange juice to a pot. Stir to combine, warm under low heat until thawed, then process using a hand blender.
Add the reserved orange pieces, cinnamon, star anise, cloves, and nutmeg. With the heat still set on low, cover, and warm for 25-30 minutes, stirring from time to time (see notes). Do not let it boil.
Add the brandy, sieve, pour into glasses, garnish and serve warm.
Notes
For a more intensive flavor, cook the Glühwein under low heat for 50-60 minutes.
Washing the oranges in hot water helps remove wax, normally coated on oranges to extend their shelf life. It also enhances their aroma.
Keep the Glühwein from boiling as this might make some of the alcohol evaporate. A temperature of between 72°C and 75°C is ideal.
Adjust the amount of sugar to suit your taste; fresh blueberries can be used in place of the frozen ones.