Dry grind all the ingredients for the rasam powder finely and store in an airtight jar.
Heat the ghee in a deep pot and add the ingredients for the tempering - mustard and fenugreek seeds, asafoetida and curry leaves and let them pop and crackle taking care not to burn them.
Lower the flame and gently saute the garlic for 10 seconds before tipping in the tomatoes. Season with salt and stir in 2 tbsps of the rasam powder. Cover and cook on a low flame till the tomatoes are mushy.
Pour in the coconut milk and continue to simmer for 5-10 minutes, stirring to prevent the coconut milk from splitting.
Toss in the chopped coriander, cook for a minute more and turn off the heat.