Sort, wash, and drain the mung beans. Place them in a heavy saucepan, along with the water, turmeric, ground coriander, grated ginger, and a quick dollop of olive oil.
Stirring occasionally, bring to a full boil over high heat. Reduce the heat to low, cover with a tight-fitting lid, and gently boil for 1 hour. The dal should be soft and fully cooked.
While this is cooking, roughly chop the spinach.
When the dal has cooked for an hour, off the heat, uncover, and add the salt. Beat with a wire whisk or hand-held immersion blender. Add the fresh spinach, cover and boil gently for 5-8 minutes more.
Combine the cumin, asafoetida, and cayenne in a small bowl.
Heat the 2 tablespoons of oil in a small saucepan or skillet over moderate to moderately high heat. When it is hot, pour in the cumin seeds and fry until they are brown. Add the asafoetida and cayenne, and fry for just 1-2 seconds more. Then quickly pour the fried seasonings into the soup. Cover immediately.
Let the seasonings soak into the hot dal for 1-2 minutes. During this time, juice the lime. Add the lime juice, and stir. Taste for salt. I often add 1/2 – 1 teaspoon more salt at the end, but it’s safer to start with less, and work up to what you need.