Heat the butter in a heavy based frying pan or heatproof casserole dish. Fry the onion over a medium heat for 3-4 minutes until soft.
Add the mushrooms and fry for a further 3-4 minutes. Cut the liver into 2cm pieces, add to the pan and cook for 5 minutes stirring occasionally.
Stir in the flour and paprika, cook for 1 minute and then gradually stir in the sherry and stock. Add the tomato puree and rosemary and continue to cook for 1-2 minutes
Bring to the boil, turn down to a simmer and cook for 2-3 minutes. Season to taste and serve.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.