Mirepoix is a posh name for what the Hairy Bikers call the 'Holy Trinity of cooking'.
All is it is a mix of chopped onion, carrot and celery in the ration of 2:1:1 and it forms the base for soups, stews, casseroles and similar. It's used all over the world in different ways and under different names. I virtually always start my soups and meaty mixes with it.
I chopped onion (stingy eyes, ouch), carrot and celery this morning and now I have a big bowlful that costs £1.20, more or less
I shall use some in some savoury pork mince I intend to cook today and the rest I shall freeze in single portions with the price on top! How would I manage this without my scales?
Here's a couple of articles that will tell you more, if you're interested.