Whole Foods charges about $4 a slice for something like this. I made two whole pies (10"x5") for about $10 total. And that was using purchased puff pastry, which is both easy to work with and tasty, but you could make your own dough (a traditional pie dough) if you prefer.
I like to share these (either whole pies or individual turnovers) with friends on camping trips, picnics, concerts, etc. These are for HSB15. Fall is in the air, and the combination of caramelized onions and mushrooms with potato wrapped in a golden pastry is really satisfying.
What You Need
1 box Pepperidge Farm puff pastry sheets
1.5# russet potatoes
1 large onion
8oz crimini mushrooms
an egg for eggwash
How to Make it
The puff pastry comes frozen. Thaw the whole box in the fridge overnight. Keep them cold as you prepare the filling.
Peel, cube and boil the potatoes (with 2t salt) until soft. Drain and roughly mash them.
Dice the onion, and saute in olive oil until soft.
Slice the mushrooms and add them to the onions. Sprinkle with S&P. Crank up the heat and saute til the onions are golden brown and the mushrooms are tender. You can deglaze the pan with a little dry sherry if you like. It adds a nice flavor and helps collect all the flavors from the pan.
Combine the onion mixture with the potatoes and let it cool to room temp. You can also make this mixture ahead and chill it overnight.
Preheat oven to 425' (you will reduce the heat to 400' after you put the pies in).
Line 2 baking sheets with parchment.
Flour a work surface and unfold one of the pastry sheets. Keep the 2nd one in the fridge.
Gently roll the square out a bit making it about 2" bigger all around. Spread half the filling evenly in the center of the dough, leaving about 3" on each side, and 1" at the top and bottom. Join the sides over the center of the pie, pinching the dough together to create a tight seal. Seal and crimp the top and bottom as well. Gently lift the pie and place it in the center of a baking sheet. Glaze with egg wash. Place the baking sheet in the freezer or fridge while you repeat the process with the second pie.
Bake the pies at 400' for about 30-40 minutes. Rotate the baking sheets halfway through. The pies should be deeply golden brown. Cool on a rack. Either serve slightly warm, or cool completely and store wrapped at room temp for up to a day. Beyond that, you should keep them in the fridge, though that will make the dough gunky. Just eat them within a day. It won't be a problem. Trust me.