Cook the potatoes and garlic in a saucepan of lightly salted boiling water for 10–12 minutes until tender, then drain. Return the potatoes and garlic to the pan and mash with the crème fraîche and sage. Season well with pepper.
Meanwhile, season the flour with salt and pepper, then press the pieces of liver into the seasoned flour to coat them all over. Heat the oil in a frying pan, add the liver and fry for 1–2 minutes on each side or until cooked to your liking.
Serve the liver hot with the garlic mash and gravy, garnished with extra chopped sage.