Put the lentils in a pot with 5 cups water, turmeric, 1 tablespoon of butter and ½ tablespoon of salt. Bring to a boil then lower the heat and simmer, covered, until the lentils are soft and falling apart, about 20 minutes.

While the soup is cooking, prepare the onion flavoring. In a medium skillet over low heat, cook the onion in 2 tablespoons of butter with the cumin and mustard seed, stirring occasionally. When soft, about the time the lentils are cooked or after 15 minutes, add the cilantro and cook another minute. Add the onion mixture to the soup and then add the juice of 1-2 limes. The soup should be a bit sour.

Just before serving, add 1 tablespoon of butter to the skillet. When foamy, add the spinach, sprinkle with salt and cook just long enough to wilt.

Serve the soup with a scoop of rice (optional) and spinach. Swirl in a spoonful of yogurt if you wish.