Heat the oil or ghee in a large heavy-bottomed pot over medium heat and add the mustard seeds, cumin seeds, dried red chili, and curry leaves. Once they begin to crackle, add the chopped onion and garlic and sauté until fragrant and lightly softened. Stir in the tomatoes, turmeric, rasam powder, black pepper, and salt, then cook for 3–4 minutes until the tomatoes begin to break down. Add the rinsed rice, cooked dal, tamarind paste, and water or vegetable stock, stirring everything together until well combined. Bring the mixture to a gentle boil, then reduce the heat, cover, and simmer for 18–20 minutes, or until the rice is tender and has absorbed the fragrant rasam broth. Turn off the heat and let it rest, covered, for 5 minutes before gently fluffing it with a fork. Finish with fresh coriander and serve hot.