Put the onions in a large bowl and cover them with cold water. Using a stainless-steel knife, peel the onions under water – to prevent tears! Put the peeled onions in clean, cold water.
Tie up the peppercorns, allspice and cloves in a small square of muslin and put into a large stainless-steel or enamel saucepan with the vinegar and salt. Bring to the boil, reduce the heat and simmer for 3 minutes. Skim any scum from the surface and discard the spices.
Drain the onions and dry well with paper towels. Bring the vinegar to the boil, add the onions and cook for 5 minutes, or until they are translucent.
Strain the hot vinegar off the onions and reserve. Pack the hot onions tightly in clean, warm, dry jars. Fill each jar with enough hot vinegar to cover the onions completely, then cover the jars with vinegarproof lids. When the onions are cold, label and date the jars and store in a cool, dry, airy cupboard for one month before eating. Use within six months.