Add into a glass bowl the mustard seeds, all-spice, salt and pepper and mix. Next add the onions and mix. Slowly pour the white wine while whisking. Finally pour the Vinegar while whisking. Cover with plastic wrap and leave for 24 hours refrigerated.

Next day add the mixture into a blender or fast-speed food processor with the blade. Blend as much as you like for a desired consistency and whether you want to have or not any bits of the mustard seeds.

Add the mustard into glass jars of about 200 ml and refrigerate upto a month.