Add the strawberries, sugar, lemon juice, and cornstarch to a medium saucepan. Place the pan over medium-low heat and bring to a gentle simmer. Cook for 5 minutes until the strawberries and have softened. Mash with a potato masher.
Cook for another 5-10 minutes until thickened. Transfer to a bowl to cool. (Pop the bowl in the fridge to help speed up the process.)**Be sure that the strawberry jam has cooled completely before assembling the pies. If the jam is still warm it will cause the butter in the crust to melt, resulting in a soggy rather than flakey crust.
Cream Cheese Filling
In a medium mixing bowl, beat together the cream cheese, egg yolk, vanilla, lemon zest, and salt until smooth and combined. Set aside.
Pie Crust
Line two baking sheets with parchment paper. Set aside. Unroll the pie crust onto a lightly floured work surface. Use a rolling pin to gently roll the pie dough and remove any creases.
Use a 3 1/2-inch heart cookie cutter to cut 10 hearts from each pie dough sheet. (You may need to re-roll the pie dough once to get all 10 cut-outs.)
Cut a small X into the center of 10 of the hearts to create a steam vent. These are the top pieces for the pies. Set aside.
Assembly
Place the remaining 10 dough hearts onto the baking sheets. Place a heaping tablespoon of cream cheese filling on the dough hearts and spread, leaving a small border around the edge. Top the cream cheese with a heaping tablespoon of strawberry filling.
Brush the border of each pie with egg white (you can use a pastry burhs or your finger). Place the top vented pie dough heat on top. Press the edges together to seal the pie. Then use the tines of a fork to crimp the edges closed.
Chill the pies on the baking sheets in the fridge for 30 minutes.
Preheat your oven to 400 degrees F. Brush the tops of the pies with egg white and sprinkle on some sugar (if using).
Bake for 20-25 minutes or until the pie crust is golden brown.
Remove from the oven and allow to cool on the baking sheets for 5-10 minutes before serving.
Notes
Store any leftovers in the fridge for up to 2 days. Reheat in a 300 degree F oven for 10-15 minutes.
You can use fresh or frozen (thawed) strawberries. Frozen strawberries contain more liquid than fresh and will need longer to cook to reach to correct jam consistency.