Chop the leftover meat into roughly 2 cm cubed pieces, removing any skin/fat as you go.
Fry the chopped onions and spread them on the bottom of a greased dish, one that the pastry sheet will completely fit over.
Fry the chopped mushrooms in the same frying pan, along with any meat that is still bloody.
the leftover meat, any leftover vegetables and leftover gravy to the
mushrooms and heat until the gravy boils to thicken it up.
Season to taste.
Add this mixture to the greased dish.
Gently unroll the pastry and place it over the dish, pushing it
against the edges of the dish and removing any excess pastry afterwards
by making cutting motions downwards on the outside of the dish with a
knife (this excess pastry can be used to make a pretty pattern on the
top of the pie).