In a large pot over medium-high heat combine the lentils and 6 cups of water. Bring to a boil, then add the spinach, ginger, turmeric, 3/4 of the green chiles, and all of the tomatoes. Reduce the heat, and simmer for 1 1/2 to 2 hours or until the lentils are extremely soft. You may need to add a bit more water during the cooking process to keep the lentils soupy.
After an hour and a half, stir in the salt. In a separate pan, heat the butter and cumin and fry until the cumin seeds start to pop. Now add the red chile powder (and asafetida if you're using it) and fry for another 30 seconds. Taste and add more asafetida if you like. Add this butter mixture to the lentils and allow to cook for another five minutes. Taste, and season with more salt if needed. I also enjoyed a touch of lemon juice added at this point. Serve topped with the cilantro and the remaining green chiles.