Pair this soup with a salad and bread for lunch or a light supper.
1. Melt 2 tablespoons butter in large saucepan over medium heat. Add onion and 1 teaspoon salt and cook, stirring occasionally, until softened but not browned, about 5 minutes. Add coriander, cumin, ginger, cinnamon, cayenne, and 1/4 teaspoon pepper and cook until fragrant, about 2 minutes. Stir in tomato paste and garlic and cook for 1 minute. Stir in broth, water, and lentils and bring to simmer. Simmer vigorously, stirring occasionally, until lentils are soft and about half are broken down, about 15 minutes.
2. Whisk soup vigorously until it is coarsely pureed, about 30 seconds. Stir in lemon juice and season with salt and extra lemon juice to taste. Cover and keep warm. (Soup can be refrigerated for up to 3 days. Thin soup with water, if desired, when reheating.)
3. Melt remaining 2 tablespoons butter in small skillet. Remove from heat and stir in mint and paprika. Ladle soup into individual bowls, drizzle each portion with 1 teaspoon spiced butter, sprinkle with cilantro, and serve.
