1/4 cup butter
2 large sweet potatoes, peeled and chopped
3 large carrots, peeled and chopped
1 apple, peeled, cored and chopped
1 onion, chopped
1/2 cup red lentils
1/2 teaspoon minced fresh ginger
1/2 teaspoon ground black pepper
1 teaspoon salt
1/2 teaspoon ground cumin
1/2 teaspoon chili powder
1/2 teaspoon paprika
4 cups vegetable broth
plain yogurt