Add blueberries to a small pot along with sugar and baking soda. Cook for approximately 5 minutes on medium heat until blueberries start to burst and form a liquid.
Mix cornstarch and water and pour the slurry into the pot. Cook for an additional 3-5 minutes until blueberry compote starts to thicken. Remove from heat and add lemon juice.
Pop Tart
Defrost puff pastry according to instructions on the package. Cut one sheet into 4 equal pieces.
Place an equal amount of blueberry compote into each cut piece of puff pastry. Fold the edges of the puff pastry on on itself and push them together with the edge of a fork. Use the fork to poke holes in the top of each pop tart.
Crack and whisk and egg in a small bowl. Using a pastry brush, brush the top of each pop tart with egg.
Bake at 350 degrees fahrenheit for 20 minutes. Remove from the oven. Let cool for 3 minutes and serve.