OK, I've not 'perfected' a recipe, but my first attempt is pretty damn good - and they're ready after less than a month. I used a mixture of white wine & rice wine vinegars, mirin & sugar to sweeten, flavoured with juniper berry (lightly crushed), peppercorns, bay leaf and root ginger, all simmered for 10 minutes, then strained out. Added to prepared onions when cold. Ready in about three weeks and kept in the fridge to avoid the 'heat bomb' effect.