In the same kadai, put the oil and when it is hot, add the mustard. When it splutter, add the dhals, red chillies broken into bits, fry till the dhal turns brown. 
Add chopped onion and curry leaves. 
Fry till onions are transparent.
Add 2 1/2 cups of water along with salt. Bring to boil. When it starts boiling, add the rawa and stir well.
Close with lid and cook on low flame till 1/2 cooked
Add Idli Podi and mix it well
Cook till all the water is absorbed and the rawa is cooked well.