Preheat your oven to 425°F (220°C).
Place the pie crust in a 9-inch pie dish and crimp the edges. Prick the bottom with a fork to prevent bubbling.
In a large bowl, combine the blueberries, sliced strawberries, sugar, cornstarch, salt, and lemon juice. Gently toss until the fruit is well coated.
Pour the fruit mixture into the prepared pie crust. Dot the top with small pieces of butter.
Bake in the preheated oven for 15 minutes. Reduce the temperature to 350°F (175°C) and continue baking for an additional 25-30 minutes, or until the filling is bubbly and the crust is golden brown.
Remove the pie from the oven and let it cool completely on a wire rack, about 2 hours. This will help the filling set.
Once cooled, arrange additional blueberries and sliced strawberries on top of the pie to create an American flag design, if desired.
Serve with whipped cream, if using.