Next time I do the recipe I will weigh the onions in advance. I must admit, I hadn't given it much thought, but it is right to question as the amount of onion can be quite different. Though I have given a rough size, so that should help.
Crucially though, I have since realised that the one ingredient measure that needs to be right is the egg. The amount of egg defines how much gram flour you will need to to make a thick sticky batter. The problem here is that if you don't have enough onion, you end up with too much batter to onion and centres won't get cooked. We only ever have X-Large eggs in our fridge and I half this for the recipe. You are looking to have just enough batter to onion to hold it together. If you find you have too much batter, add another onion.
There shouldn't be any need to prepare the onions the day before, it really isn't necessary. I have produced near exact results at home as to the restaurants bhaji without doing so. As long as you work the onion stage by stage, it should be pliable enough when the mix is ready to be formed and cooked.
The final part is forming the baji. Take a handfull of the mix and form a ball but but don't squeeze it to hard, you need to roll or smooth it gently in wet hands from one hand to another. If all has gone well, you will be holding a surprisingly weighty bhaji that you feel may fall apart in the oil, but of course it won't.
The second cook process only serves to reheat the bhaji and help crisp it up. If you do second cook, gently squeeze or pull the bhaji to break it very slightly. You will get a much crispier bhaji if you use the second cook method.