Preheat the oven to low. Halve, peel and thinly slice the onion.
Heat half the olive oil in a large frying pan over high heat and fry the onion, stirring, for 1 minute. Lower the heat, cover and cook for 6 minutes, or until the onion is just tender and is beginning to brown, then transfer to a plate and keep warm.
Meanwhile, strip off the sage leaves and chop enough to give 1 tablespoon, then put aside.
Season the liver on both sides with black pepper. Add the remaining oil to the frying pan and, when very hot, fry the liver for 1–2 minutes on each side, until it changes colour. Transfer to a serving dish to keep warm.
To make the sauce, bring the vinegar and 2 tablespoons water to the boil in the pan, stirring and scraping up the browned residue. Reduce the heat and stir in the mustard and cream.
Return the liver and onions to the pan, reheat very gently in the sauce for 1–2 minutes, sprinkle with chopped sage and serve.