I guess you all know that there is no such thing as an Onion Bhaji in India; it is another UK first folks probably based on Pakoras. I posted this recipe on another thread but has got lost somewhere in the ether. I don't have this written down as I just do them as required. They are very simple and with a little practice they are very quick to make from scratch Here goes. :

To make two palm sized bhajis you will need the following:
Heat oil to about 160 degrees C
1 use a large Onion mild Spanish or Chilean are best, cut in half lengthwise and then in to 3 - 4 mm slices. Remove the core before you slice, and also remove the smaller inner bits because they burn easily.
2. Put in a bowl and mix with about half teaspoon of salt and a pinch of sugar (optional)
Leave for about a minute and then mix 2 tblsp of Gram (Besan) flour with the onions thoroughly.
3. Leave for a few minutes while you make the flour mix, this allows the flour, salt and sugar to release some of the juices from the onions and neutralise the acidic taste.
4. Mix about 4 tbsp of besan flour with a quarter teaspoon of Garam Masala (your own is best), and about half teaspoon of whole Cumin seeds (you can use Ajwain instead but they go better with the Spinach version). Add a large pinch of Bicarb or baking powder, or Epson salts and mix the lot together well. You can add some potato, rice, or corn flour to make it more crispy; about 1/2 teaspoon should do it. I use potato flour.
6. Add the spice flour mix to the Onions and mix around well, leave for 30 seconds.
7. Add a little water, a little at a time, while you mix until you get a consistency like double cream or thick creamy yoghurt.
8. Scoop up a handful of onions and press them in to shape and put in the oil, do this for the rest of them.
9. Cook until a light golden brown and the onion is nice and moist.
10. Serve with salad and raita.
Cooking notes. Any batter fried product will continue to cook and darken after you remove it from the hot fat, so don't be tempted to cook it to the colour you finally want.
The raising agent e.g. baking powder, will start to lose its effectiveness after about 10 to 15 minutes, so you can't really make this in advance.
You can add a little chili powder and turmeric to the flour mix (not onion mix) if you wish, but be careful not to use too much turmeric as it burns very easily in hot oil.
If you do not plan to use them immediately don't store in a container, just leave them out on a plate and warm them in the oven (not microwave!) when you need them.
Good luck; sorry I am not so precise but this is a real bir style jobby. Once you get the knack and a spice ratio that you like you will have no problems.
Cheers
CP