Preheat the oven to 375 degrees Fahrenheit and prepare a 9-inch pie dish.

In a large bowl, combine chopped strawberries and rhubarb. Add sugar, brown sugar, and corn starch. Mix until the fruit is well coated and the mixture begins to look slightly thick.

Add vanilla extract, lemon juice, and salt. Stir again until evenly combined and the fruit mixture looks glossy and evenly mixed.

Place one pie crust into the pie dish. Press it gently into the base and sides, making sure it fits evenly. Trim any excess dough along the edges.

Pour the prepared fruit filling into the crust. Spread it evenly to ensure consistent baking.

Place the second pie crust over the filling. Either cover completely or cut into strips for a lattice pattern. Trim excess dough and seal the edges by pressing them together. Crimp edges as desired.

Beat the egg in a small bowl and brush it over the top crust. This helps create a golden, shiny finish during baking.

Place the pie in the oven and bake for 45 to 50 minutes. Check that the crust is golden brown and the filling is bubbling through the top openings.

Remove the pie from the oven and allow it to cool at room temperature for at least 2 hours before slicing. This helps the filling set properly.