Wash dal thoroughly. Soak 1 hour, drain and rinse.
In a large soup pot, combine dal, turmeric, and salt with 6 cups of water. Bring to a boil. Stir and cover pot with lid slightly ajar. Reduce heat to medium. Cook until softened, about 15 to 20 minutes. Set aside.
While dal is cooking, prepare spice mixture. Heat oil in a skillet over medium heat. Add cumin seeds. Heat until seeds are fragrant, turn brown, and start crackling. Add onions. Cook until dark brown, stirring frequently. Mix in ginger and garlic paste. Cook until mixture turns dark brown, stirring occasionally to prevent onions from burning or sticking to the pan. Add red chilies, coriander, garam masala, and tomato purée. Cook another minute, stirring frequently. Remove from heat and
add spice mixture to soup pot with cooked dal.
Place pot over medium heat, cook 3 to 5 minutes without the lid, stirring occasionally.
Serve hot with rice or naan (Indian bread). Garnish with chopped cilantro.