Heat the mustard oil to medium, then pour it over the besan. It will bubble and needs to be stirred continuously until it cools.
Add the salt, carom seeds, asafoetida, garlic, chillies and coriander and mix well.
Add the water, a little at a time until you have a thick pouring batter. Cover with film and set aside for 2½ hours.
to cook
Once the batter has rested, it will have risen a little and is ready for cooking. Add the onions and mix well, ensuring the onion is well coated in the batter.
Heat the frying oil in the kadai until quite hot. Test the oil by dropping a small piece of batter into it. If it sizzles and rises to the top without immediately colouring it is the right temperature.
Using your hands, take a handful of onion mixture and squeeze it into a small flat pattie, no bigger than your palm.
Carefully drop the bhaji into the oil and repeat the process, making sure you do not overcrowd the pan or allow it to cool. You may need to cook these in batches, depending on the size of your kadai.
The bhaji are done when the batter is crispy golden brown on the outside and cooked through. The onions should still have some crunch.
to serve
Turn bhaji onto absorbent paper to drain the oil, then serve immediately.
notes
The onions should be halved, then sliced. The article how to slice onions shows a simple technique to safely and quickly prepare the onions.