just in case I'm doing something really wrong.... I boil the onions and other ingredients for a few hours (5 in this case). I then allow the mixture to cool overnight and bring it back to the boil the next day, I then blend it before adding it to the hot spiced oil. Is this right?

The recipe I use is actually from this website. I appreciate that people know what they are doing on here so I have not modified anything from that recipe. I don't remember who posted the recipe but I do remember that it was someone who got the recipe first hand from someone that worked in a BIR. Just to clarify, what exactly have I modified?

I will defienitely chop the onions next time. I've just had a look a  few base recipes and some do say to quarter or even half them. Do most people blend before adding it to the spiced hot oil in a sepearate stock pot?

i only 1/4 the onions and simmer for 2 hrs before blending. i usually blend straight off the heat using a hand blender in the base pan (here https://curry-recipes.co.uk/curry/index.php?topic=4026.msg36484#msg36484. never had any problem getting the base smooth.

my only thought not base related but using the hand blender generally - they work better in a deep jug (to work they have to suck the ingredient down through the plastic shroud and into the blade). when blending base (if i'm not measuring it for volume when i use a deep jug) then i tend to tip the pan to one side to create the depth needed.

I'm not expecting it to be that smooth but most of the time if i put a spoon to the bottom of the pan I can lift out a mashed up onion gloop while the top will be very watery. It doesn't mix together. I think I will try a jug blender and chopping the onions to see if it helps. But for now I've got 15 portions of lumpy sauce!