How to Make It
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.
In the same skillet, heat 1 tablespoon each of butter and olive oil. Add the sliced onions and cook over moderate heat, stirring occasionally, until golden brown, about 20 minutes. Season with salt and pepper. Add the Madeira to the skillet and cook until almost evaporated, about 3 minutes; keep warm.
In another skillet, melt the remaining 1 tablespoon of butter. Add the patties and cook over moderately high heat, turning once, until browned, about 8 minutes. Lower the heat to moderate and cook, turning once, until no trace of pink remains, about 8 minutes longer. Spoon the onions onto a platter, top with the sausage and serve.