2 large, fat sweet potatoes (about 1 1/2 pounds), peeled and cut into eight 1/2-inch rounds
3 tablespoons olive oil, plus more for oiling the grill grates
Kosher salt
One 15-ounce can black beans, rinsed
1/2 cup frozen corn, thawed
1/3 cup finely diced pepper Jack cheese
5 tablespoons stone-ground cornmeal
1 large egg white
2 scallions, thinly sliced
2 tablespoons mayonnaise
4 lettuce leaves