Put the oil in a large pan and set over a medium—high heat. When hot, put in the asafoetida, mustard and cumin seeds. As soon as the seeds begin to pop, a matter of seconds, add the flour. Turn the heat to medium—low. Stir a few times with a whisk until the flour turns a shade darker and is a pale brown.

Slowly add the stock, stirring with a whisk as you go. When well mixed and thick, add the passata, mixing as you do so, then add the curry leaves, turmeric, cayenne pepper, salt and sugar. Bring to the boil. Lower the heat and simmer for 25 minutes.

Turn off the heat and leave the soup in the pan as long as possible so that the spices release more of their flavours. Push through a sieve and reheat before serving.