2 Place onions in a bowl, cover with boiling water and allow to cool in the water.
3 Remove onions and squeeze out of skins.
4 Mix onions with the sea salt in bowl and let stand overnight, covered with a kitchen towel.
5 Rinse jars (each about 500 ml) (approximately 16 ounces) and their matching lids with boiling water, then leave upside down to dry on a kitchen towel. Rinse onions thoroughly with cold water and drain well.
6 Divide onions, mustard seeds and cloves among prepared jars.
7 Bring white wine vinegar and sugar to a boil in a pot. Add balsamic vinegar and cook for 1 more minute.
8 Pour hot liquid into jars and secure lids. Place upside down for 5 minutes, then turn right side up and let cool completely. Refrigerate for up to 2 weeks.
Tip: To preserve for longer, process jars according to manufacturer's instructions (or visit http://nchfp.uga.edu/).