4 tablespoons balsamic vinegar (or use cider vinegar)
3 tablespoons sugar
1 tablespoon salt (or to taste)
Sit outside where a breeze is blowing, and peel the onions.
Slice thinly on a wooden board. Best is to stay outside even if you need a parka, because inside you'll cry a lot over so many onions.
Use the largest pot you have, like a soup pot, and heat. When hot, add about an inch of oil (2 1/2 cm). Let oil heat well.
Slowly add onions to the pot. The good thing about a deep pot is that there is less spattering. You might have to add in batches if you peeled a very large amount, and you might have to add more oil.
Stir every now and then. This will take a long time, so have a nice paperback nearby --
Fry the onions with patience until they soften. Add a few splashes of water if they threaten to catch. You could add more oil, but you cannot keep adding oil as the end result will be too oily.
When at long last the onions have collapsed and actually starts to colour slightly, add the vinegar, sugar and salt. Stir through. This improves the flavour -- add more or less, to taste.
Read your book and keep stirring as you don't want to burn the lot at this late stage.
When the onions are done to your taste, pull off heat and cool.
The least bulky way to freeze them is to put them in ziplock bags. You will know how much sauteed onions you use per average dish -- rather put in too little because you can always use more than one pouch. If you freeze too much in a bag, some might go to waste.
Push out air, seal, label, cool completely, and freeze.