Grind spices: Place the mustard and coriander seeds in a food processor and grind them finely. Alternatively, you can do this in a coffee grinder or using a mortar and pestle.
Orange zest: Wash the oranges with hot water, dry them well and grate the zest. Place it in a small bowl, add the sugar and rub everything together with the fingers until well mixed.
Orange juice: Halve and juice the oranges. Pour the juice in a small pan, bring to a boil and let cook until reduced to 100 ml/ slightly less than ½ cup. It took me about 20 minutes to achieve that, but it depends very much on the size of the pan, so keep checking.
Let the juice cool slightly, sieve over the ground mustard.
Combine: Add the orange zest-sugar mixture, grated ginger, salt, and vinegar, and mix everything very well together.
Store: Transfer the mixture to a clean bowl and leave uncovered, in the kitchen, overnight. Transfer to clean, small jars and refrigerate.