In 4-quart saucepan, over high heat, bring broth, water, salt, garlic, cayenne, and sweet potatoes to a simmer. Reduce heat to low and simmer 15 to 20 minutes, until sweet potatoes are fork tender. Using potato masher or whisk, mash about 1/2 of the cubed sweet potatoes into broth to thicken. The more you mash the thicker the texture. Add kale and push down into hot soup. Once all kale wilts, simmer on low 15 to 20 minutes, or until kale is tender and tasty. Undercooked kale can have a slightly bitter flavor that mellows with cooking. Season with salt and pepper, and serve hot with a squeeze of fresh lemon. The lemon is optional, but really brings out the flavor of the kale and sweet potato.
Super-nutritious and tasty, this fat-free soup is incredibly easy and fast to prepare. No butter or oil is needed when you’re using the delicious combination of satisfying sweet potatoes and flavorful kale, both on many “super foods” lists.