Comment on I can't believe it's necessary to ask the question...
irishPotato@sh.itjust.works 4 days ago(Click saving) this guy (and myself) move the potatoes to another ceramic, add celery to the carrot-onion mix & chop that and leave it under the roast
Gronk@aussie.zone 3 days ago
Bit more info, this is a common combination for stocks, soups and stews in French cuisine, traditionally 2:1:1 ratio onions to carrots and celery.
In Italian cuisine the essentially same thing is Sofrito, there is also the Cajun Holy Trinity which varies ingredients but is for similar purposes.
This is kind of a staple for a lot of quality cooking, you should at least make a stock from scratch at some point in your life and you’ll see why these combinations are so important if you do so.