Heat the oil in a heavy 4-quart saucepan over medium-high heat.

Add the onion, gingerroot, garlic, fennel and cumin seeds, and red pepper flakes and cook, stirring, until the onion begins to soften, 2-3 minutes.

Add the lentils and broth and bring to a boil. Reduce the heat to medium-low, cover the saucepan, and simmer until the lentils are soft and the soup is thick, 45 minutes to 1 hour.

Transfer half the soup into a blender and blend until smooth (or use an immersion blender on half the soup). Return the blended soup to the saucepan with the unblended soup and mix well. Add salt to taste.