Break the tamarind block up as best you can and soak in hot water for an hour or longer. This will soften up the dry tamarind. It should be squishy.
Massage the pulp to help separate seed and skins. I followed mum's advice to wear rubber gloves as tamarind is quite acidic.
Mash and squeeze the pulp to release a thick liquid of the flesh and water. Mum usually uses a colander or sieve to squeeze the pulp against for this step however the most recent time I made the ketchup, I was at a friends and found her steaming set a great help – a large pan with small colander-sized holes in the base that fits snugly on top of a large saucepan – much more stable than mashing into a colander or sieve balanced in a pan or bowl!
Depending on how well you've extracted flesh from the seeds and skins, you might want to re-soak the remnants in a smaller volume of hot water and make a second pass of mashing and squeezing. I do usually do this.
You should end up with a large quantity of thick liquid.
If you used a colander for the previous step, you may wish to strain the liquid through a sieve to get rid of any remaining lumps of skin or seed but if the liquid looks smooth and lump-free, don't bother.
Discard the seeds, skins etc.
In a large pan heat the oil. Add the cumin seeds and asafoetida powder.
When the seeds splutter, pour in the tamarind liquid and all other ingredients except the garam masala.
Allow it to boil briskly, stirring from time to time.
Taste and adjust salt, jaggery/sugar and chillies to reach your preferred balance of sweetness, acidity and heat.
If the liquid is too thin continue to heat to reduce volume and thicken up. Note, this ketchup is not intended to be really thick and gloopy but of a pouring consistency.
Add garam masala and stir in.
Take off the heat and allow to cool.
Pour into sterilised, airtight bottles or jam jars.
Notes
* The quantity of these ingredients should be adjusted during cooking, according to taste. The tarter the tamarind, the more salt and sugar it will need.