You can also roast the sweet potato slices.

Peel the sweet potato and cut into 1/2 –inch-thick rounds. (Choose thick, round sweet potatoes over long, skinny ones.) Heat the cooking fat over medium heat in a large skillet, swirling to coat the bottom of the pan. When the fat is hot, add the largest slices from the middle of the potato, laying them out in a single layer in the pan. (Cook these in batches, if necessary). Cook until fork-tender and browned, 3 to 5 minutes on each side. Sprinkle with salt and pepper. Let cool, then stack a burger and fillings between two slices of sweet potato and server.