Normally, we pressure cook Toor dal for Sambar and Rasam together. If you are cooking Rasam alone, ensure you cook with enough water to get a well cooked dal.
Soak tamarind in water and extract the juice.
Once the dal is well cooked and mashed, in a bowl mix the dal, tamarind juice along with chopped tomatoes. Add salt, curry leaves, and coriander leaves. Add the required amount of water.
Pressure cook for 1 whistle. Crush garlic along with the red chilies and curry leaves. This adds more flavour to the rasam.
Heat a pan with oil. Temper with the seasonings. Then, add rasam powder. Fry for a min and gently add the boiled rasam. Bring to a boil and remove.