How to Make It
In a large, heavy saucepan, heat 1/4 cup of the olive oil until shimmering. Add the onions and cook over moderate heat until softened and lightly browned, about 10 minutes.
In a large pie plate, season the flour with salt and pepper. Dredge the slices of liver in the flour, shaking off any excess. In a large skillet, heat the remaining 1/4 cup of olive oil until shimmering. Add the liver slices and cook, turning once, until browned and just cooked through, about 5 minutes. Transfer the liver to a platter, cover loosely with foil and keep warm.
Pour off any excess oil from the skillet. Add the stock and boil until reduced by half, about 5 minutes. Add the onions, sage and lemon juice and cook for 2 minutes; season with salt and pepper. Spoon the onion-and-lemon sauce over the liver and serve.