Combine water, turmeric, coriander, bay leaves, garlic and cayenne pepper in a large stock pot and bring to a boil. Add lentils, reduce heat, cover and simmer until tender, stirring occasionally, about 45 minutes.
Heat oil in a heavy large skillet over medium heat. Add onion, ginger, cumin and mustard seeds. Cook until onion begins to brown, stirring often, about 10 minutes. Mix in lemon juice. Stir onion mixture into lentils. Add salt. Ladle soup into bowls and sprinkle with cilantro or mint. Serve with lemon wedges. Garnish each bowl with a dollop of plain yogurt, if desired.
Recipe tip: If soup is thin, simmer until reduced to desired consistency.