Combine all filling ingredients except cornstarch in a saucepan, mashing with a potato masher (or the back of a fork) until just slightly chunky.
Bring to a boil, and let boil for 45 min - 1 hour, stirring regularly. Once mixture has reduced to a jam, remove from heat and let cool. If your pie crusts need to defrost, take them out now.
Preheat oven to 400 F. Line a baking sheet with parchment paper.
Mix 1 cup jam with cornstarch.
In a separate bowl, beat the egg.
Roll out the pie crusts, one at a time, to approximately a 9" square. Cut each pie crust into three 3" strips.
Spoon jam onto one side of each rectangle, leaving the edges uncovered.
Brush the edges of the rectangle with egg, then fold the uncovered side of the rectangle over the jam side, and seal the edges tightly.
Press the edges with a fork, then brush the top of the poptart with egg.
Bake poptarts for 15-18 minutes.