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.