Chop 1 small onion, 1 carrot and 1 stick celery into small dice. Place in a saucepan. Add 3 cups cold water, 1 bayleaf, a few parsley stalks and a sprig of thyme. Bring to a boil. Cover and simmer for 30-40 minutes. Remove from heat and leave to cool. Strain through a sieve. Use as required. Stock will keep in the fridge in a covered container for 2-3 days. Alternatively freeze in covered containers.