Has anyone ever done something like this- in a slow cooker, for 8-10 hours kind of thing? It seems to me that a slow-cooked base could reach a real depth of flavour- or would it suffer from over-cooking?

The onions may become a little to sweet DO

Right. I was hoping it would add a smokeyness to it too, but-! Cheers MA

Not sure on the smokiness side but when I cooked my base onions for longer 5hours+ on a low heat they became too sweet

Righto! A word from the wise! That kind of KO's my plan to leave a pot of base on low on the stove overnight too cheers

I think you need to cook at a certain temperature to get the bitter compounds in onions converted to milder, more flavoursome types. Seems like real Indian restaurants tend to boil the crap outta their onions to achieve this so not sure slow cooking is gonna achieve anything other than ending up with a pan full of stewed onions.

BIR French onion soup praps!!!

Yes, years ago I tried it as a back up, because you don

Hi DO, bit late on this reply, I have cooked Jb's base fully to speck, also in a pressure cooker. And on a slow flame with fine chopped onions for 5 hours.

Very little difference in all methods in the final curry, my personal preference was the slow cooked base, deeper in taste and made a great curry. 

I knocked up a jb base last week, after adding the tadka garlic time was not on my side so I dumped it all into a slow cooker left it for 8 hours for a meal later
.Wow what a surprise even tastier and a richer aroma
Scientifically how that happened I am not sure  I expect Einsteins tutors here will explain ;D
I know some Birs keep it on for ages
Someone please try this and share the result      "eureka moment" for me and  mrs Ox