Preheat the oven to 375°F (190°C). Place the pie crust into a 9-inch pie dish and crimp the edges as desired.

In a large bowl, combine the cherries, blueberries, sour cream, granulated sugar, flour, salt, vanilla extract, and lemon juice. Stir gently until the fruit is evenly coated.

Pour the filling into the prepared pie crust and spread it into an even layer.

Dot the top of the filling with the small pieces of butter.

If desired, brush the exposed crust with the beaten egg and sprinkle with coarse sugar.

Bake for 45 to 55 minutes, or until the crust is golden brown and the filling is set with gentle bubbling in the center.

Remove the pie from the oven and let it cool completely on a wire rack for at least 2 hours before slicing. This helps the filling firm up.

Serve at room temperature or chilled.