Spinach and Tomato Dal (Indian Lentil Soup)

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 lentils are tender, about 1 hour.

Heat 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 seeds; cook and stir until onion is soft, about 5 minutes. Stir spinach, tomato, and cayenne pepper into onion mixture; cook until spinach wilts and tomato is tender, about 5 minutes.

Stir cooked lentils and cilantro into spinach mixture; bring to a simmer and cook for 5 minutes. Adjust salt and water to taste and texture preferences.