Heat two tablespoons of oil in a medium sized soup pot over medium high heat. Add ½ teaspoon of mustard seeds and let it sizzle. Add ½ teaspoon of each turmeric and chili powder. Add 1 ½ cups of water, mung dal and season with ½ teaspoon salt. Stir everything. Cover the pot and let it come to boil. Uncover once it is boiling and let it cook until the foam on top subsides, about 7 minutes. Cover the pot and let dal cook for another two minute or until tender.
In a separate wok or a frying pan, heat remaining 3 tablespoons oil over medium high heat. Add remaining mustard seeds and cumin seeds and let it sizzle. Add onion and sauté it until translucent, 3 to 4 minutes. Add garam masala, dhana-jeeru, green chili, ginger and garlic. Sauté everything for two more minutes. Add tomato paste and sauté for a minute. Add spinach and ½ cup water. Cover the pan and let spinach cook until slightly wilted, 2 to 3 minutes. Season with ½ teaspoon salt, add kasuri methi and let spinach cook until it is fully cooked.
With a blender or an immersion blender, puree the spinach until smooth. Transfer the pureed spinach to wok or frying pan again and combine it with mung dal. Let it simmer for 5 minutes covered with a lid.