In a large pot, mix 7 cups water, lentils, turmeric, 1 tablespoon of the butter, and salt. Bring to a boil, then reduce heat and simmer, covered, until the lentils are very soft (about twenty minutes). Puree with an immersion blender (if you don't have an immersion blender, frankly, I wouldn't bother blending it. Just mash it around with a fork or potato masher). Salt to taste, if needed.

While the lentils are cooking, chop the onion. In a skillet over low heat cook the onion in 2 tablespoons of the butter along with the cumin and mustard seeds, stirring occasionally. When the onions have softened, 8 - 10 minutes, add the cilantro and cook for a few seconds before removing from the heat. Add the whole thing to the soup, then add the lemon juice a bit at a time, until the flavor has a tang that you like.

Just before serving, add the last of the butter to the skillet, when melted, toss in the spinach/chard/kale and a big pinch of salt. Stir well, and cook just long enough for the greens to collapse.

To serve, put a scoop of rice in each bowl, then ladle the soup over the top, sprinkle on the spinach, and finish with a big dollop of yogurt.