I just finished making this base again. Some tips..
I pureed the onion mix in a food processor, using one ladle full of mix at a time. I added some of the sieved onion liquid to loosen it up and help the food processor a bit. There should be no lumps in this, so if you have lumps, puree it again.
I'm now going to make some Vindaloo with this, so I'll let you know how I go. Cheers.