Sweet Potato Burgers with Lentils and Kale

This is a hearty, nutritious burger, inspired by one of my favorite trios of vegetables to use for a fall-to-winter soup. Use any hearty green you like or have on hand, including chard, beet greens, turnip greens, or mature spinach. Likewise, if you have a leftover roasted sweet potato, use it instead of the steamed potato. The burgers are delicious topped with any of the yogurt sauces. To veganize these burgers, my favorite substitute is a