Put the lentils in a large pot with 7 cups water, the turmeric, 1 tablespoon of the butter, and 2 teaspoons salt. Bring to a boil, then lower the heat, cover and simmer, until the lentils are soft and falling apart, about 20 minutes or so. Puree this mixture with a hand blender. Add more water until the soup is the consistency you like, then taste and add more salt if needed. Keep the soup warm.
While the lentils are cooking, prepare the onion. In a skillet over low heat cook the onion in 2 tablespoons of the remaining butter along with the cumin and mustard seeds, stirring occasionally. When the onions are softened, roughly 10 to 15 minutes. Add the onion mixture to the soup, then add the juice from the lemons, until the soup has a nice tang to your liking. Add more salt to taste at this point if needed.
Just before serving, add the last of the butter to the skillet. When hot add the spinach and a good pinch of salt. Stir well and cook just enough for the spinach to collapse.
Serve by placing a scoop of rice in each bowl, then soup, spinach, and a dollop of yogurt.