One of my favourite things to ferment lately: mirepoix. I had a bunch of celery that wasn't going to last much longer, so this is how I saved it. Thus is super useful for soups and pasta sauces with a lot more complexity in flavour than unfermented mirepoix. I basically ferment it until I need it, then throw the remainder in the fridge, but 2 weeks is probably a good rule of thumb. This one is 1/3 onion, 1/3 celery, and 1/3 carrot, plus a sweet pepper, mixed with 2% salt by weight.