Heat 3 tablespoons oil in a large saucepan over medium-high heat. Add chopped onion, minced garlic, 2 teaspoons ginger and ¼ teaspoon salt; cook, stirring often, until the onion starts to soften, about 5 minutes.

Add 1½ tablespoons curry powder and 1 tablespoon turmeric; cook, stirring constantly, until fragrant, about 1 minute.

Add 8 cups broth, 1⅔ cups lentils and the remaining ¼ teaspoon salt; bring to a boil over medium-high heat.

Reduce heat to medium-low; simmer, stirring occasionally, until the lentils start to soften, about 15 minutes.

Add cubed sweet potato and chopped carrots; simmer, stirring occasionally, until the sweet potatoes and carrots are tender, 14 to 16 minutes.

Meanwhile, heat the remaining 2 tablespoons oil in a small saucepan over medium heat. Stir in 1 teaspoon paprika and ½ teaspoon fennel seeds; remove from heat. Transfer to a small bowl.

Working in 2 batches, transfer half of the lentil mixture to a blender. Secure lid on blender and remove the center piece to allow steam to escape. Place a clean towel over the opening. Process until smooth, about 45 seconds. (Use caution when pureeing hot liquids.) Return the mixture to the large saucepan. Repeat with the remaining lentil mixture.

Add well-stirred coconut milk to the pureed lentil mixture; bring to a gentle simmer over medium-low heat.

Stir in 1½ tablespoons lemon juice. Season with additional lemon juice to taste.

Ladle the soup into 6 bowls and drizzle with the reserved paprika-oil mixture. Garnish with cilantro, if desired.