Soak the tamarind in warm water for 20 to 30 mins.
Squeeze the pulp from the soaked tamarind. Strain and keep aside.
In a dry grinder or coffee grinder, powder the cumin seeds, whole black pepper and garlic to a semi fine consistency.
Heat oil in a pan. Crackle the mustard seeds first.
Add the curry leaves, red chilies & asafoetida and fry for some seconds until the red chilies deepen their color.
Fry on a low flame so that the spices don't burn.
Than add the tomatoes and saute until the tomatoes soften.
Then add the semi coarsely powdered cumin, black pepper and garlic along with turmeric powder.
Stir and then add the tamarind pulp. Add water and stir well. Season with salt.
Just let the entire rasam come to a gentle simmer uncovered on low heat.
Then switch off the fire and add chopped coriander leaves.
Serve rasam hot as a soup or with steamed rice.
Lime or Lemon juice: Even though the sour taste of tamarind is different than lime or lemon, you can still add 1 to 2 tablespoons of lime or lemon juice. Add the lime or lemon juice when the rasam is finished simmering. You can vary the quantity depending on the sourness you prefer.
Tamarind paste: Add 1 teaspoon tamarind paste and mix it evenly with the warm water. If the sour taste is less, then you can add a bit more.
Make ahead and storage:
Freezing: In the freezer, rasam stays good for a month. Freeze the portion you want without adding coriander leaves. While serving gently warm or heat it and sprinkle some coriander leaves.
Refrigeration: In the refrigerator, you can store rasam for 2 to 3 days.