In a large heavy saucepan cook the onions very slowly in the butter to a soft translucency. It does not matter if they begin to turn golden; it is better that they do, since the sugar in the onions becomes more flavoursome as it caramelizes. However, they must not burn. And burnt onion skins should not be used, as they will add nothing but bitterness.
When the onions are soft, add the m