Rasam is an important part of traditional South Indian menu. There are many various recipes in different regions of India. Rasam is known as Charu in Andhra Pradesh and Saaru in Karnataka.
This soup is based on plain water, lentil stock, or a combination of both. Lentil stock (also mentioned as dhal water in Indian recipes) is a liquid strained after cooking lentils. It is added to give a nice flavor and make soup nutritious. To my taste, toor dhal (yellow lentils) and chana dhal (black Indian chickpeas) stocks are the best for rasam.
Tomatoes (prime) and tamarind (secondary) create sweet and sour background for other flavors. Those other flavors are multi-layered by spices, added to the soup in different forms on different stages of cooking. Garlic, fresh curry leaves, black pepper, and asafetida play an important role among other ingredients. Some heat from chilies makes all the flavors sound and colorful, intense.
It is believed that Rasam is a by-product of Sambar, another important dish of the Southern Indian diet. There is a story about a chef who made this soup at the end of one of the Southern Indian Maharaja’s grand party using sambar left overs. In Sanskrit, rasa means taste and also refers to juice.
Mix all ingredients for the soup base, bring to boiling, reduce the heat, and simmer for 10 minutes, no lid.
Measure all ingredients for tarka. Pound pepper corns, cumin seeds, and garlic to crush them into paste. Heat the oil in a large sauté pan till smoking hot and reduce the heat to medium. Add mustard seeds and red chili. Cook till mustard seeds start popping. Add lentils and cook for another 30 sec. Add fenugreek, asafetida, turmeric, and curry leaves and cook for another 10 sec, constantly stirring them in hot oil. Mix hot tarka into the soup base.
Simmer the soup for another 10 minutes. Chop cilantro leaves for serving soup.
Rasam is served with rice or with flat bread as a soup. There are commercial spice mixes for this soup called rasam powder, but I encourage you to ground your own. Only fresh spices make this soup really great. You can add other ingredients to the rasam base, one or two at a time — ginger, perl onions, lentils, vegetables, etc. — to make ginger rasam, onion rasam, and so on. Tomatoes are the main souring agent in this soup. If you add more tamarind, it becomes tamarind rasam.
Use slow cooker to make lentil stock. In a pot, cover 1 part of lentils with 4 parts of cold water, and add a pinch of salt. Cook lentils on low for 6-8 hours, strain, and keep cooked lentils and the stock refrigerated separately.