Heat the oil in a small saucepan over medium heat.
Add the ginger, chilli, cumin, asafoetida and garam masala, stir and cook gently for 1 minute to release the flavours and toast the spices.
Add the water, tamarind paste and brown sugar and stir until the sugar has completely dissolved.
Reduce the heat and simmer gently for 15 minutes until the sauce turns thick and syrupy – it should be the consistency of sweet soy/ketchup manis (ie. like very runny honey). Taste and adjust the tamarind to suit.
Cool the sauce before using. The sauce will thicken as it cools.