It took me a long time to appreciate the importance of a good stock - partly becuase I didn't know how to make one. Previously, I would chuck any old trimmings in a pot and boil them for too long, thinking myself thrifty. But there's no point being thrifty if the reward is flat muddy-flavoured liquid. So I now pick and prepare ingredients with more care and a good stock has become vital to our kitchen.
I usually prepare a double batch and freeze pint-size portions for use over the next month or so. This basic recipe is from Deborah Madison's Vegetable Soups book and is the one I prepare most often.
Heat the oil in a pot then add the onion, carrot, celery, and spring onion.
Stir frequentyly over medium-high heat until they begin to take on colour and glaze the pan.
Add the rest of the ingredients and about 2 litres of cold water. Bring to boil, then reduce heat and simmer, partially covered, for 30 minutes.
Carefully strain the stock into containers, leaving any muddy residue in the bottom of the pan if there is any.
You can add various other vegetable trimmings to this base, but I often find I prefer the base recipe on its own. If you do add extra ingredients - especially strong-flavoured ones like fennel - consider how this will work with the dishes you have planned.
In particular, you should always avoid adding:
Many of the above will make the stock bitter, or, in the case of beetroot, a strong colour. Incidentally, if you'd like the stock more yellow-looking (similar to many chicken stocks) you can add clean onion skins to the pot with the onions. They don't affect the flavour, but they make the stock more yellow.
The golden rule is: no sad, mouldy, or old vegetables. Use fresh vegetables you actually want to eat (though you can choose the more knobly less beautifully-formed ones).