Trim any excess fat off the hearts and remove the tubes (they're very chewy otherwise!) then enlarge the openings to enable easier stuffing.
Peal the onion & garlic, then finely chop them along with the cleaned mushrooms.
Melt the 15g of butter in a sauté pan then gently brown the onions and garlic for a couple of minutes, followed by the mushroom for another couple, then let it cool down in a mixing bowl. Add the other stuffing ingredients and mix well.
Spoon the stuffing into the hearts then wrap two rashers of bacon around each of them and hold in place with string.
Put 25g of butter into the sauté pan then blend in the cornflour before adding the cooking sauce liquids. Bring to the boil, stirring well, simmer for a couple of minutes and set aside.
Heat the oil in the pressure cooker, carefully brown the hearts all over then pour in the cooking sauce.
Increase the heat and bring up to pressure. Cook for 25 minutes then allow to depressurise by itself.
Serve with vegetables and mash (sweet potatoes mash is great with it!)