In a bowl, mix the mustard powder, flour, salt, and turmeric (if using).
Gradually whisk in the water until smooth and free of lumps.
Add the vinegar and stir well.
Pour the mixture into a saucepan and add the sugar.
Cook over medium-low heat, stirring constantly until the mixture thickens (about 10–15 minutes).
Remove from the heat and allow the mustard to cool slightly.
Spoon into clean, sterilised jars and seal.
Refrigerate for at least 24 hours before using to allow the flavours to develop.