When you make stock, do so in quantity, then boil down to concentrate and freeze in useful quantities. To darken the colour of the stock, if required, leave the skin on the onions.
Place all the ingredients in a large saucepan and add cold water to cover. Bring to the boil, skim and simmer for 2 hours, topping up with water and skimming as necessary.
Strain the liquor through a fine sieve. Taste and if the flavour isn’t strong enough, boil to reduce the stock down.