Add the sour orange juice to the jar making sure to cover every shallot slice
Close the lid and place the jar next to an indirect source of heat for about an hour or more.
Notes
Though I used sour oranges for this trick, you can use vinegar as well. Simply make sure to pour enough juice or vinegar to cover your shallots.You can preserve those shallots in that jar in the fridge for a couple of days. You could also try to add it to your marinades and sauces. It will surely brighten them up.