Here is a more or less complete picture of everything that you would need in order to construct a pie of an upside down nature. Since it's an upside down apple pie (I did mention it was apple pie, right?), you totally need some apples. Four of them, apparently. Apples of some kind of variety that look not unlike the apples seen here.
You will also need various forms of sugar, at least one kind of flour, some spices of some sort, some butter that should be melted but the picture was taken with the butter in an un-melted state.
Oh, and some pie crusts. Two of them. You could make them yourself, if you have that kind of time. But some people are far too busy for that kind of thing, because there are more adventures to be had than the making of a single upside down pie!