These wonderful treats are the result of my efforts to replicate toaster pastries, which are a great vice of mine. I finally nailed them.
2-2/3 cups flour
1/2 tsp salt
2 tsp granulated sugar
1 cup cold butter, cut into 1/2" pieces (2 sticks)
1/2 cup cold water
2 - 3 cups cooled Apple Pie Filling
Whisk flour, salt, and sugar together in a medium bowl. Add the butter pieces, making sure to coat them in flour with a spatula. Use a pastry blender to cut the butter into the flour until most of the butter is pea sized and all the flour is incorporated into the butter.
Sprinkle the water over the flour and butter mixture, using the spatula to moisten all the flour and scrape down the sides and bottom of the bowl. Gather the dough into a ball and divide it into two equal sized disks. You may need to chill the dough in the fridge for 15 minutes if the room is warm and it is very sticky.
Preheat oven to 400 degrees.
Turn a disk of dough out onto a work surface dusted with flour. Flour a rolling pin and gently roll out the dough, making sure to flip the dough over and rotate it every couple passes with the pin to keep it from sticking. Sprinkle flour on the work surface, dough, or pin if these areas seem to be sticking.
Once rolled out, cut the dough into roughly equal sized squares (see pic below). Don't worry about the rough edges, they will be taken care of later.
Place half of the dough squares on a cookie sheet (see pic below). Spoon the pie filling (to taste, start with a spoonful or two) into the center of each square. Top with a square of roughly the same size. Pinch the edges shut on all sides, tucking any rough edges in (using a fork to crimp helps). Pierce the top 3x with a fork to let steam escape (so they won't be as likely to leak). At this point they can be frozen unbaked and popped in the oven on a moment's notice. Just make sure to wrap them up airtight and separated with wax paper so they don't stick to each other. Repeat with the second half of the dough.
Bake in preheated oven for 15 minutes or until golden brown. They are best fresh, but can be reheated later with some success. Makes about a dozen 3-4" pastries.