Peel (only if you don't like the skin), cube and sautee for about 10 minutes the eggplant in a pan with a little olive oil.
Meanwhile, boil the sweet potatoes till tender. Once cooked cut the sweet potato in cubes.
In a large bowl, combine the eggplant and sweet potato and smash them with the back of your mixing spoon. Add the egg, oregano, flour or breadcrumbs, salt and pepper.
Shape into patties about 3'' in diameter. In an oiled pan fry patties until browned, about five minutes on each side.
Serve on buns with condiments and toppings of your choice.