Separate the coriander leaves from the stalks. Reserve the leaves for garnish, and chop the stalks finely; set aside.
Add curry powder to a dry saucepan and toast over medium heat for 2 minutes. Add oil and stir; powder will sizzle. Add onion, apple, garlic, ginger and coriander stalks. Season with salt and pepper, and cook gently 5 minutes.
Peel and grate the sweet potatoes. Add to sauce pan with broth, lentils and milk. Simmer, covered, 20 minutes or until lentils and sweet potatoes are tender. Blend smooth with an immersion (stick) blender.
Stir in lime juice, adjust salt and pepper and serve, garnished with reserved coriander leaves.