Put the onions in a large bowl and tip over a kettle of boiling water. Leave for 20 seconds then pour into a colander, return to the bowl and pour over lots of very cold water. The skins should now peel off very easily.
Once peeled, layer in a bowl and sprinkle each layer with salt. Cover and leave overnight or up to 24hrs.
Meanwhile pour the vinegar and sugar into a large saucepan with the peppercorns, mustard seeds and bay leaves. Put the lid on the pan, bring to the boil then remove from the heat and set aside to allow the spices to infuse.
The following day rinse the onions well and pat dry.
Pack the onions into sterilised Kilner® Jars then use a jug to pour over the vinegar, including the mustard, peppercorns and bay, completely covering the onions. Seal and allow to mature for 6 weeks.