Note to self: yes, a fine dice of onion, carrot and celery does have a name (I knew it!). It's called a mirepoix. I am writing it here so that the next time I forget what's it's called (and start getting twitchy because I hate knowing that there is an answer and not being able to remember what it is) I will be able to come here and find out.

Hopefully I won't forget that it's here...