Cut the butter into ½-inch cubes and freeze for 30 minutes to keep it extra cold.
1 cup unsalted butter
In a large bowl, sift together the flour, salt, and sugar. Add the cold butter and use a fork or pastry cutter to mix until the texture becomes grainy. Gradually add ice water, 2 tablespoons at a time, mixing until the dough begins to come together.
2 ½ cups all-purpose flour, 1 teaspoon salt, 1 teaspoon sugar, 6 to 8 tablespoons ice water
Transfer the dough to a floured surface and shape it into two 6-inch circles, kneading as little as possible. Wrap each in plastic wrap and refrigerate for at least 1 hour.
Meanwhile, heat the butter in a skillet over medium heat. Add the rhubarb and ¼ cup of sugar, cooking for about 5 minutes until slightly softened. Stir in the strawberries, remaining sugar, vanilla extract, and lemon juice. Cook for another 2-3 minutes, then remove from heat and let cool.
1 ½ tablespoons brown sugar, 1 ½ pounds chopped fresh rhubarb, 1 ½ pounds strawberry, 1 tablespoon lemon juice, 1 teaspoon vanilla extract, 1 tablespoon salted butter
In a small bowl, dissolve the cornstarch in 2 tablespoons of water, then mix it into the cooled fruit filling.
4 tablespoons cornstarch
On a floured surface, roll out both dough circles to about 12 inches in diameter. Fit one into a 9-inch pie plate, trimming any excess to leave a 1-inch overhang.
Pour the rhubarb filling into the pie crust. Cut the second dough circle into thick strips and arrange them in a lattice pattern over the filling. Fold the overhanging edges from the bottom crust over the lattice edges and pinch to seal. Use your fingers to create a fluted edge.
In a small bowl, whisk the egg whites with 1 tablespoon of water. Brush the mixture over the crust, then sprinkle with brown sugar.
1 egg white, 1 tablespoon water, 1 cup granulated sugar
Preheat the oven to 350°F (180°C). Bake for 1 hour or until the crust is golden brown and the filling is bubbling. Allow the pie to cool on a wire rack for at least 4 to 5 hours before slicing.
Serve with vanilla ice cream or whipped cream. Enjoy!