Melt butter in a large, heavy-bottomed pot over medium-high heat. Add sweet potatoes, carrots, apple, and onion; cook and stir until onion is translucent, about 10 minutes.
Stir in lentils, salt, ginger, pepper, cumin, chili powder, and paprika, then pour in broth. Increase the heat to high and bring to a boil. Reduce the heat to medium-low, cover, and simmer until lentils and vegetables are soft, about 30 minutes.
Purée soup in the pot with an immersion blender until smooth. Increase the heat to medium-high and cook until simmering, about 10 minutes. Add water as needed to thin the soup to your preferred consistency.