Peel and roughly chop the onions into thin wedges. Scatter in a deep roasting tin with the bay leaves, peppercorns, cider, ham and butter then cover with foil.
Roast for about 1 hour, until onions are very soft.
Tip onions into large saucepan, then splash some of the veg stock into the roasting tin to scrape up all the juices and add to the pan with the rest of the stock. Simmer until slightly reduced, season to taste and serve with crusty bread.