Prepare the crust: Pulse flour and salt in a food processor. Add cold butter and pulse until mixture resembles coarse crumbs. Slowly add ice water, one tablespoon at a time, until the dough just comes together. Divide dough in half, flatten into disks, wrap in plastic, and chill for at least 30 minutes.

Prepare the filling: In a large bowl, gently combine the chopped rhubarb and quartered strawberries. Add the granulated sugar, flour, vanilla extract, and salt. Toss lightly until the fruit is evenly coated. Set aside while you roll out the crust.

Assemble the dessert: Preheat oven to 400°F (200°C). Roll out one disk of dough and carefully fit it into a 9-inch pie dish. Trim the edges. Pour the fruit filling into the crust.

Top the pie: Roll out the second dough disk. You can use it as a full top crust, cutting slits for venting, or cut into strips to create a lattice top. Crimp the edges to seal the top and bottom crusts together.

Bake the dessert: Brush the top crust lightly with the beaten egg wash. Optionally sprinkle with coarse sugar. Place the pie on a baking sheet to catch any drips. Bake for 20 minutes at 400°F, then reduce the temperature to 375°F (190°C) and bake for another 30-35 minutes, or until the crust is golden brown and the filling is bubbly.

Cool and serve: Allow the dessert to cool on a wire rack for at least 2-3 hours before slicing to ensure the filling adequately sets. Serve warm or at room temperature, perhaps with a scoop of vanilla ice cream.