Add all dry ingredients together and mix: mustard seeds, salt, white pepper, all spice. Add shallot / onion and mix. Add white wine vinegar and mix. Add white wine and mix. Cover with plastic wrap and refrigerate for at least 8 hours.
Add mix to blender or food processor. Blend until thickness and chunkiness desired. Taste and add brown sugar if too bitter for your taste.
Store in a small mason jar in refrigerator for up to 2 weeks to 2 months. See notes.
