Add the roughly chopped onions, celery, carrots to a large pot (these should be the base of all vegetable socks). The add some of the vegetable discard from your freezer bag, or other vegetables as desired. Add fresh herbs as desired.
Add water to the pot using the general rule that 4 cupsof vegetables make 2 litres of stock. Season with salt and pepper.
Bring to the boil, and then cover and allow to simmer for an hour or more (the longer the better).
Strain the stock using a sieve and allow to cool, before storing in an air tight container, in a fridge or freezer.