Place the onions in a large heatproof bowl and pour over boiling water to cover. Leave to cool. Once cool, trim roots and tops and peel. Sprinkle the salt over the peeled onions, stir and leave overnight.
Next day rinse the onions well and dry with kitchen towel. Place the vinegar and honey into a large pan and gently heat just to dissolve the honey into the vinegar, but do not boil.
Pack the onions into clean, sterilized jars. Pour over the hot vinegar mixture to fill the jars, and check there are no air pockets. Seal the jars and leave to cool. The onions will be ready to eat after about 1 month or better if kept for 2. Once opened store in the fridge.