Preheat your oven to 375°F (190°C).

In a large bowl, combine the flour, salt, and sugar. Add the chilled butter and mix until the mixture resembles coarse crumbs.

Gradually add ice water, one tablespoon at a time, mixing until the dough comes together. Divide the dough in half, shape into discs, wrap in plastic wrap, and refrigerate for at least 1 hour.

In another bowl, combine the rhubarb, strawberries, sugar, cornstarch, vanilla extract, lemon juice, and cinnamon. Toss until the fruit is well coated. Set aside.

Roll out one disc of dough on a floured surface to fit a 13x18-inch baking sheet. Place the rolled dough onto the baking sheet and trim any excess.

Pour the fruit mixture into the crust, spreading it evenly.

Roll out the second disc of dough and place it over the fruit filling. Trim and crimp the edges to seal. Cut slits in the top crust to allow steam to escape.

Brush the top crust with the beaten egg and sprinkle with coarse sugar.

Bake for 45-50 minutes or until the crust is golden brown and the filling is bubbly.

Allow to cool for at least 30 minutes before slicing and serving.