Grind tomatoes, garlic, ginger, and shallots (if using) with black peppercorns, cumin, and coriander seeds to a fine paste.
Add water, adjust to a soup consistency, and bring to a boil.
Add all the spice powders and salt.
Add tamarind extract if needed.
Let it simmer for about 20 minutes.
Strain the contents through a sieve to get a clear soup.
Temper with mustard seeds, red chilies, and curry leaves.
Video
Notes
Save time with a pressure cooker: If you're short on time, you can pressure cook for 1 whistle for the rasam to speed up the cooking process while still developing great flavor.
Adjust the tamarind to taste: Tamarind extract is optional if your tomatoes are already tangy. Taste the rasam before adding it and adjust the amount to suit your preference.
Rasam powder is optional: This recipe is flavorful with just the freshly ground spices. However, you can add a little rasam powder for extra depth of flavor if you like.
For a thinner rasam: If you prefer a light, clear, restaurant-style rasam, strain it through a fine-mesh sieve after it has simmered. This removes the solids while keeping all the delicious flavors.
Don't skip the tempering: A final tempering with mustard seeds, dried red chilies, and curry leaves is essential for authentic flavor. Use ghee for a rich, aromatic finish or coconut oil for a traditional Kerala-style taste.
Let it rest before serving: Allow the rasam to sit for 5–10 minutes after tempering. This gives the spices time to infuse, resulting in an even more flavorful bowl.