Boil a large saucepan of lightly salted water for the pasta.
Heat 12.5g butter with the oil in a frying pan until hot.
Fry the liver for 1-2 minutes per side until browned and firm.
Season the liver, transfer to a plate, and keep warm.
Add port to the pan and boil until reduced by half.
Stir in the vinegar and cook for a few seconds.
Pour in the stock, boil, and reduce the liquid by half.
Cook the pasta in the boiling water until tender.
Drain the pasta and toss with the remaining butter and sage.
Season the pasta and divide it between two warm plates.
Place the liver on top of the pasta.
Whisk crème fraîche into the sauce and spoon it over the dish.