This is a basic dal (Indian lentil soup) recipe that my mom always makes. You can substitute different vegetables and greens and follow similar procedure. This is a hearty and healthy dish that leaves you satisfied without overfilling you. You can substitute spinach for other dark greens like mustard greens, kale, or swiss chard. The cook time will vary. Serve with steamed rice.
Bring lentils and 4 cups water to a boil in a saucepan. Stir in salt. Reduce heat to medium-low and cook at a simmer 10 minutes. Skim off any scum that forms and continue to simmer until the lentils are tender, about 1 hour.
Heat the oil in a separate saucepan. Cook mustard seeds and cumin seeds in hot oil until they start to splatter, 2 to 3 minutes. Add onion, garlic, and turmeric to the seeds; cook and stir until the onion is soft, about 5 minutes. Stir spinach, tomato, and cayenne pepper into the onion mixture; cook until the spinach wilts and the tomato is tender, about 5 minutes.
Stir the cooked lentils and cilantro into the spinach mixture; bring to a simmer and cook for 5 minutes. Adjust salt and water to taste and texture preferences.