Rinse the toor dal in cold water until the water runs mostly clear. Soak in fresh water for 15 minutes, then drain.

Add the drained dal, 4 cups water, and turmeric to a large pot. Cover and bring to a boil over high heat (2–3 minutes). Reduce heat to low and simmer for 30 minutes until the dal is soft and breaking apart. If it still feels hard, cook another 5 minutes. Some liquid should remain — if there's too much, drain and reserve it for later.

About 5 minutes before the dal is done, heat 1 teaspoon ghee in a medium skillet over medium heat. Add the cumin seeds and toast for 30 seconds until fragrant.

Add the chopped onion and green chili. Sauté for about 3 minutes until the onion is soft and fragrant.

Add the tomatoes and press them with a spoon to release their juices. Add the ground cumin, ground coriander, Kashmiri chili powder, and salt. Stir well and cook for 3–5 minutes until the tomatoes break down into a thick base.

Add the dal base to the pot of cooked lentils. Stir to combine, cover, and cook together for 5 minutes. Add up to ½ cup of reserved cooking liquid to reach your preferred consistency.

In the same skillet (no need to wash), heat 1 tablespoon ghee over medium heat. Add the mustard seeds. When they begin to sputter and pop, add the cumin seeds and dried red chilies if using. Let them sizzle for 20 seconds, then add the Kashmiri chili powder and stir for 5 seconds — remove from heat immediately so the ground spice doesn't burn.

Pour the hot tadka directly over the dal — it will sizzle and crackle on contact. Garnish with chopped cilantro and serve hot with rice or flatbread.