Heat the oil in a saucepan over medium heat. Add the cumin seeds, ginger, cayenne pepper, fennel seeds, asafoetida powder and garam masala; cook and stir for about 2 minutes to release the flavors.
Stir the water into the pan with the spices along with the sugar and tamarind paste. Bring to a boil then simmer over low heat until the mixture turns a deep chocolaty brown and is thick enough to coat the back of a metal spoon. This should take 20 to 30 minutes. The sauce will be thin, but it will thicken upon cooling.