Preheat oven to 180 °C.
Melt the butter in a hot pan, add the onions, peppers and mushrooms and fry gently until the vegetables are soft and the excess liquid has cooked away.
Add the beef strips and cook for 6 minutes.
Add the cream and the full contents of the sachet of KNORR Beef Stroganoff Dry Cook-in-Sauce. Stir well, then simmer until the sauce has reduced by half – the mixture should be quite creamy and not too dry.
Remove the mixture from the pan and place in a 23-cm round pie dish.
Lay the puff pastry sheet over the filling and remove the excess pastry around the edges. Press down lightly on the rim to seal.
In a bowl, beat the egg with a fork, then brush the pastry with the egg wash.
Prick the pastry using a fork, then use the left-over pastry to create shapes to decorate the top of the pie.
Bake at 180 °C for 20–30 minutes, or until puffed and golden.
Serve hot with a side salad of rocket and mixed leaves drizzled with olive oil.