Combine strawberries, champagne, and sugar in a saucepan, and bring to a boil. While it's heating, use a potato masher or a fork to mash the strawberries. This will be easier to do as they heat.
Once boiling, let boil for 45 min - 1 hour, stirring regularly, until reduced to a thickened filling.
Once thickend, remove from heat and let cool.
If your pie crusts need to defrost, take them out now.Preheat oven to 400° F. Line two baking sheets with parchment paper.
Mix 1 cup of the reduced strawberry mixture with cornstarch.
In a small 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 the reduced strawberry filling onto half the length 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.
Cool completely before adding icing.