Rinse the toor dal and masoor dal under cold water until it runs clear, then soak for 15 minutes and drain.
Combine the soaked dals, 3 cups water, and turmeric in a medium pot. Bring to a boil, skim foam, then reduce to low, cover, and simmer until the dal is very soft and mashable, 25–30 minutes. Lightly mash with a spoon or whisk.
Scrape the tomato-onion masala into the cooked dal. Stir in 1 cup hot water to reach a pourable but slightly thick consistency, then simmer 5–7 minutes to meld. Season with the salt.
For the tadka, heat 2 tbsp ghee in a small pan over medium heat until shimmering. Add the cumin seeds and let them crackle, 10–15 seconds. Add the sliced garlic, asafoetida, and dried red chilies; fry, stirring, until the garlic just turns light golden, 30–60 seconds. Immediately pour the hot tadka over the simmering dal, cover the pot for 1 minute, then stir in the kasuri methi and garam masala.
Finish with the lemon juice. Serve hot, garnished with cilantro, alongside steamed rice or roti.