Once you have fermented Idli batter and ready to steam the idli, heat 1-2 tbsp oil in a sauce pan.
Add in dry urad dal (you can rub the dal between a towel to clean it or wash/soak it for 15 min and pat dry it on a towel before sautéing.)
Keep cooking/mixing urad dal in oil on low/med heat. Once urad dal starts to deepen in color and forms aroma, add in mustard seeds and asafetida.
Once mustard seeds start to pop, add in curry leaves, grated ginger and chopped serrano (green chilis). Mix well and saute for few seconds.
Add in grated carrots, peas and corn along with ½ tsp salt and red chili powder. Mix well and cook for 2-3 minutes.
Turn the heat off and mix the cooked mixture in fermented batter. Mix well, add water if needed to get idli batter consistency. It's time to steam the idlis. Start bringing water to boil in your idli steamer.
Once water is boiling, oil the idli trays (each time you use them) and place it in the steamer so that the tray starts to warm up.