Okay, first thing you have to do is get that mental picture out of your mind. It's pie as in Shepherd's Pie, not Apple Pie.
You will need:
Frying pan or wok
Oven or toaster oven big enough to take a pie
One frozen pie crust
One medium yellow or white onion
One medium to large tomato (and store-bought is okay, for once)
Small can of black olives
Prepare the frozen pie crust according to the package directions for a one-crust pie. (In other words, pre-bake it so it doesn't become a soggy mess.) Slice the onion into fine pieces*. Over medium to medium-high heat, fry the onions in a little olive oil, turning constantly until soft and almost caramelized. (You can turn the heat down so as to prevent burning - it will take longer.) Spread a thin layer of dijon mustard in the pie crust. Dump the onion in on top. Wedge the tomato (cut from the center) and arrange atop the onions. Grate cheese on top; spread some black olives on the cheese. Bake for 5-10 minutes in the oven (at the temperature directed by the pie crust directions) until the cheese is melted and the tomato warmed through.
I have to warn you, it looks horrible. Gallery of Regrettable Food level horrible. But it tastes so good that one gourmet-loving friend of mine who was brave enough to sample my leftovers declared that he'd always be up for it.
Serves four people, unless they're hungry.
*Onions are notorious for making people cry. Three things that help are 1) soaking the onion in ice water for 5-10 minutes before slicing; 2) keeping your knife extra sharp; and 3) having a well-ventilated area. But you will still get irritated eyes.