Preheat the grill to medium-high, direct heat.
Peel the sweet potatoes and slice into 1/4 inch thick slices.
Coat the slices with olive oil.
Form the beef into patties.
Grill the potatoes and burgers, 3-6 minutes each side or until the burgers are cooked as you like and the sweet potatoes are softened and have some nice grill marks.
While things are grilling, mix the cilantro, lime zest and juice, oil and salt in a large bowl.
Add the sweet potatoes to the dressing and mix.
Place the burgers on a bun with a slice of feta and some lettuce.
Enjoy!