Place the sliced onions in a saucepan. Add white vinegar and water. Boil open while stirring now and again for 5 minutes. Strain and discard vinegar water. To the same saucepan with the onions, add the balsamic vinegar, sugar and Seasoned Sea Salt. Boil open for ±45 minutes until thick and jammy.
Sterilise clean glass bottles with boiling water. Spoon the onions into the glass jars and close tightly. Turn the jars upside down to sterilise the lids. Store in the fridge.