Pressure cook the mangoes with a cup of water.
Grind the cooked mango pieces with water to a smooth consistency, filter to make sure there are no chunks present. Use half a cup of water to rinse the blender jar and add to the filtered mango water.
Add a cup of water, turmeric, ginger and the Serrano pepper to a bowl and simmer for 3-4 minutes.
Add salt and the mango water and simmer for 2 minutes.
Heat oil in a pan over medium flame. Add the red chiles and mustard seeds, when it splutters add the cumin followed by asafetida and curry leaves. Fry for 30 seconds.
Add it to the rasam.
Garnish with cilantro and serve with steaming bowl of rice and veggie curries.