Pressure cook the lentils with green chillies, ginger, onion and tomatoes till mushy. It takes about 15 minutes. You can also cook this in a pan, but it will take longer. To expedite, you can soak the lentils for about an hour before cooking. Cool, mash with the ladle and keep aside.
Now to prepare the tempering keep all the ingredients at hand. Heat a pan. Add ghee. When it melts, add cumin and allow it to sputter. Add garlic, dry red chillies and asafoetida in quick succession. Now add in chilly powder. Take special care not to burn the tempering. Now tip in the prepared dal (lentils). Mix well and simmer on low heat for 2-3 minutes. Add some water if it is very thick.
Garnish with coriander leaves and lemon juice and serve hot with boiled rice. You can also have it as a soup if you so desire. Your dal tadka is ready to serve.
You may use a combination of lentils eg. pigeon peas with red lentils or split green gram with red lentils. All are equally tasty.
You can also serve this dal as a soup. It is quite delicious.