This rich pie takes some time to cook, but we promise it's worth the wait. The "aah Bisto" aromas that fill your house as it cooks will get everyone to the table on time.
Preheat oven to 170 degrees centigrade.
In a large bowl, mix together the tomato paste, water and Bisto Best Beef Gravy Granules. Stir until the Granules dissolve.
Add the steak, mushrooms, onions, garlic, thyme, and Guinness and stir well.
Transfer to a deep baking dish (around 30cm x 22cm) set on a large baking tray. Cover the dish with foil and put in the oven. Bake the filling, stirring occasionally, until the meat is very tender, around 3.5-4 hours.
Beat egg and 1 tsp of water together in a small bowl and set aside.
Roll puff pastry out on a lightly floured surface to a 0.5cm thickness. Make sure it ends up 5cm wider than baking dish.
Remove the baking dish from the oven and uncover.
Roll the puff pastry loosely around a rolling pin and then unfurl it over the filling. Crimp the pastry around the edges to make the pie airtight.
Brush the pastry with the egg wash to help it colour and crisp as it cooks.
Return the baking dish to the oven and bake until pastry is puffed and deep golden brown, about 40 minutes.
Serve with an extra drizzle of Bisto Best Beef Gravy (made to pack instructions).