Heat a frying pan until hot and add 25g/1oz butter.
Season the liver with salt and freshly ground black pepper and add to the frying pan. Fry on each side for 2-3 minutes, until cooked to your liking. Remove from the pan and set aside.
Add the red wine to the pan and bring to the boil, scraping to release the sticky, caramelised cooking juices from the base of the pan.
Reduce the liquid by half, then add the beef stock and boil for a further 5-6 minutes.
Add 25g/1oz butter and stir in. Remove from the heat.
Heat a deep-sided frying pan until hot then add the remaining 25g/1oz butter and the bacon pieces. Fry until crispy then add the cavelo nero and stir-fry for 3-4 minutes, until wilted.
Add the lemon juice and season well with salt and freshly ground black pepper.
To serve, place the cabbage in the centre of two plates. Top each with the liver and a spoonful of crème fraîche. Spoon over the red wine gravy.