Preheat the oven to 180C.
Make the stuffing by melting half the butter in a sauté pan, add the onion and cook over a medium heat for 2-3 minutes, add the mushrooms and continue to cook for 2 minutes. Allow this mixture to cool.
Mix the cooled onion and mushroom mixture with the sausage meat, parsley, seasoning and egg and mix well. Make the sauce by heating the oil in the sauté pan, add the flour and cook for 1-2 minutes.
Add the stock, canned tomatoes and wine, bring to the boil, reduce the heat and simmer for 4-5 minutes. Spoon the stuffing into the prepared hearts. Wrap the hearts with 2 rashers of bacon and secure with string.
Heat the remaining butter in the pan and brown the hearts on both sides and transfer to an oven proof dish. Pour the sauce over the hearts and cook in the preheated oven for about two hours or until just tender.
Remove the string, slice and return to the sauce, stir through the chopped parsley and serve .