You can use the left over to make pie.
Defrost some ready made puff pastry. Line them inside pie dish. This can be the individual small one or the big shared one.
Put the stroganoff or casserole inside puff pastry, but make sure you do not put too much liquid or it will be a mess later on.
Then cover the top of the pie.
Bake according to the pastry direction and you've got yourself a beef pie.