2oz (50g) butter
2lb (900g) beef fillet, cut from the centre of the fillet
2 large onions, sliced
4oz (100g) mushrooms, sliced
Salt and freshly ground black pepper
14oz (397g) packet of frozen puff pastry, thawed
Beaten egg, to glaze
Melt the butter in the small roasting tin. Trim any fat off the fillet, lift into an AGA roasting tin and roll in the butter.
2, 3 and 4 oven AGA: Hang the tin on the top set of runners in the Roasting Oven and cook for about 20 minutes, turning the meat over once so that it is browned both sides. Lift out onto a plate and cool completely. Add the sliced onion to the juices in the tin and stand on the floor of the Roasting Oven to cook, stirring from time to time until golden, add the mushrooms and cook for 2 more minutes. Lift out of the oven, season and leave to cool.
Roll out the pastry to a rectangle 14 x 16 inch (35 x 40cm) depending on the size of the meat. Place half the mushroom mixture down the centre of the pastry then place the meat tin on top, flat side uppermost. Season well. Cut a 1.5 inch (3.5cm) square from each corner of the pastry for decoration and keep on one side. Fold the pastry around the meat so that it is sealed inside. Turn over and stand on a baking sheet. Brush liberally with the beaten egg. Use the left over pastry to decorate the top with leaves or lattice. Chill in the refrigerator for up to 24 hours before baking.
2, 3 and 4 oven AGA: With the grid shelf on the floor of the Roasting Oven, cook for about 25-30 minutes until pale golden brown. Make a good gravy from the reserved onions and mushrooms to serve with the beef.
Taken from The AGA Book by Mary Berry