In a skillet, heat 1/2 tablespoon of the olive oil until shimmering. Add the chopped onion and cook over moderate heat until softened, about 4 minutes. Transfer the onion to a bowl. Add the pork, apple, egg, cracker crumbs, sugar, sage, nutmeg, cayenne and zest to the bowl; season with salt and pepper and mix well with your hands. Shape into 8 patties about 1/2 inch thick.