Heat oil over medium heat in large, heavy soup pot. Add onion and cook, stirring often, until tender, about 5 minutes. Add garlic, ginger, 1 teaspoon mustard seeds and 1 teaspoon cumin seeds and cook, stirring, until fragrant, about 30 seconds to a minute.
Stir in turmeric, squash, red lentils and 6 cups chicken broth. Turn up heat, add salt to taste and bring to a boil. Reduce heat to low, skim off foam, cover and simmer 35 to 45 minutes, until squash and lentils are tender. Taste and adjust salt.
Purée soup using an immersion blender or, working in batches, in a blender. Return to pot if using blender. Add black pepper, taste and adjust salt, and heat through.
Just before serving, heat butter over medium heat in a small skillet or saucepan and add remaining 2 teaspoons mustard seeds and 1 teaspoon cumin seeds. Cook, stirring, until seeds are lightly colored and fragrant, about 2 minutes. Stir into soup.
Serve with a generous dollop of yogurt and a sprinkling of cilantro if desired.
Recipe from New York Times Food, Martha Rose Shulman