Melt butter in large pot on 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 heat to high and bring to a boil. Reduce 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 heat to medium-high and cook until simmering, about 10 minutes. Add water as needed to thin to desired consistency.