You have to,
1. Wash the dal several times.
2. In a pressure cooker pan add dal, green chilli, asafetida, salt and 3 cups of water and pressure cook for 3 whisles. (you could boil further if you want the the dal to be thick)
3. Heat oil in a non-stick pan
4. Add broken red chilli and mustard
5. Once it pops up, add onions and sauce till it turns light brown
6. Pour this over the cooked dal.
7. Serve with rice and roti.