Pressure cook yellow lentil with 1/4 cup water. Mash it nicely and keep it ready.
Now add cooked dal and 2 cups of water into it and add some salt and mix it well. ILet it simmer for a minute. Remove from the flame and keep it aside.
Take a small frying pan and add 1 tbsp ghee and mustard seeds to it. Once the seeds crackle, add few urry leaves and let it sizzle for for seconds. Add this in rasam and garnish it with some coriander leaves.