Make the Pie Dough: Follow the directions in my post "How to Make Perfect Pie Crust" through step 7, and under step 7 follow the directions to par-bake your crust. Let your par-baked crust cool completely to room temperature before filling it with the fruit filling and proceeding with the bake. While the pie dough is par-baking, you can start on your fruit filling.
Make the Strawberry Blueberry Filling: In a small bowl, whisk together the ingredients for thickening the filling. Set aside. Add the fruit filling ingredients to a medium saucepan - the blueberries, strawberries, 2 tbsp sugar, vanilla bean paste, lemon zest, and lemon juice. Cook over medium heat for 10 minutes, stirring lazily. The berries will start breaking down and become quite juicy. Sprinkle the cornstarch/sugar mixture over the fruit and stir well to combine. Cook, stirring constantly, until the mixture comes to a simmer - this will happen quickly, 1-2 minutes. Once it's simmering, cook for just 1 minute. Remove from the heat and spread cooked fruit filling onto a large baking sheet and let cool completely in the fridge or at room temp.
Bake the Pie: Pre-heat your oven to 400° F. Beat the egg in a small bowl and brush over the edges of your pie crust. Brush the edges of the crust with egg wash and sprinkle on coarse sugar. Add cooled fruit filling to the room temp par-baked crust. Bake for 46 to 52 minutes, checking the crust for browning around 30 minutes. Add a piece of foil over the top of the pie to prevent the crust over browning. The filling will appear set and the crust will be deep golden brown when finished. Let the pie cool at room temp for at least 1 and 1/2 to 2 hours before slicing and serving.
Make the Star Topping: While the pie is baking and cooling, make the cutout topping. Roll out the second disc of dough to between 1/4 and 1/8 inch thick. Cut out stars or any desired shapes. Place on a parchment lined baking sheet and brush with egg wash and sprinkle with cinnamon sugar. Bake at 425° for about 10-14 minutes. This will vary depending on the size of your cutouts. They should be deeply golden brown and feel crisp when tapped with a fingertip.
Serve + Store: Once ready to serve the pie, top with your pie crust stars and serve with vanilla ice cream! You can also let the pie cool to room temp, then cover and refrigerate before serving the next day. (I personally prefer it a bit warm from the oven!) This is best sooner rather than later, but leftovers can be stored, well-covered, in the fridge for 2-3 days.