I finally have light at the end of the Onion Bhaji tunnel. The key to getting them like the BIR is to cook them the day before or at least 8 hours before use. Once part cooked they need to sit at room temperature for 7 - 8 hours. This allows all the spices to get absorbed into the onions which in turn soften without burning. All of the cakey rawness of initial experiments is gone, replaced by light and melt in the mouth bhajis. My last try was:
1 large onion (300 grams) chopped very finely in food processor
1 potato (100 grams) chopped very finely in food processor
0.5 tsp salt
1tsp ground coriander
1 tsp ground cumin
2 tsp cumin seeds
handful of chopped fresh coriander
All the ingredients went in the bowl for an hour to draw some of the moisture out of the onions. Then I added gram flour and a little bit of water to make a thick but runny batter.
I deep fried them in vegetable oil at 140 degrees for around 4 minutes so they were just going golden. Then I removed them from the oil, put them on a plate and stuck them in a cupboard until that evening.
The second fry through only took about 2 minutes at 150 degrees.
They tasted fantastic but I think the onion and potato was cut a bit too thin so the result was more like a fishcake than a bhaji. Nest time I'll cut them by hand.
Also, I added a tsp of baking powder to the mix just before frying which meant as soon as they hit the oil they swelled up. Again, I don't think this brought a lot to the party so I'll give it a miss next time. A couple more tries and I think I'll have the perfect Bhaji recipe / method!