Combine all the ingredients in a glass jar or bowl. Cover with a clean cloth and let stand at room temperature for 2-3 days. The seeds will plump but there will still be liquid in the bottom of the jar.
Put the ingredients, including the liquid. into a blender and blend until smooth. Add a few tablespoons of water if the mustard is too thick. Taste for seasoning and adjust as necessary. Transfer to a glass jar and refrigerate until ready to use.
Notes
Storage: The mustard will keep for up to 6 months refrigerated.