For the rasam powder/podi:
Dry fry the ingredients till light brown and to release the aroma (about 3-4 mins)
Remove and blend/process till fine.
For the rasam:
Add 1-2 tbsp rasam powder and all ingredients for the rasam into a pot with 1/2 liter of water.
Simmer (do not boil) until heated through.
In another pan, heat oil and quickly fry chilies and mustard seeds.
Add into the rasam liquid and off the heat.
Stir these in and also the coriander leaves.