Melt the butter in a large saucepan over medium-high heat. Add chopped sweet potatoes, carrots, apple and onion. Stir and cook the apples and vegetables until the onions are soft and golden in colour, about 10 minutes.
Stir in lentils, ginger, ground black pepper, salt, cumin, chilli powder, paprika and vegetable stock. Bring the soup to the boil; reduce heat, cover and simmer until the lentils are soft, about 30 minutes.
Working in batches, pour the soup into a blender or food processor and blend until smooth. Alternately, you can use an immersion blender and puree the soup in the saucepan.
Warm the soup before serving, about 10 minutes. Add water as needed to thin the soup to your preferred consistency. Serve with yogurt or crumbled feta.