PREPARE THE FRUIT: Hull and slice the strawberries. Wash, trim, and chop the rhubarb into ½-inch pieces. Place both in a large mixing bowl.
MIX THE FILLING: Add sugar, cornstarch, lemon juice, and salt to the fruit. Stir until everything is evenly coated, then let the mixture rest for 30 minutes to draw out the juices and help the thickener start working.
ROLL OUT THE CRUST: Roll out the bottom crust and place it into a 9-inch pie dish. Leave the edges hanging over the sides. Keep the top crust chilled until ready to use.
ASSEMBLE THE PIE: Use a slotted spoon to transfer the fruit mixture into the prepared crust, leaving any excess liquid in the bowl. Spread the filling evenly in the dish.
ADD THE TOP CRUST: Roll out the top crust. You can place it as a full sheet or cut into strips to make a lattice. Arrange it over the filling, then trim and crimp the edges to seal.
BRUSH AND SPRINKLE: Beat the egg with the cream or milk to make an egg wash. Brush it evenly over the top crust. Sprinkle the crust with sugar for texture and shine.
BAKE THE PIE: Place the pie on a baking sheet and bake at 400°F (200°C) for 20 minutes. Then reduce the heat to 350°F (175°C) and continue baking for 40 to 45 minutes, until the crust is golden and the filling bubbles visibly through the lattice or vent holes.
COOL COMPLETELY: Transfer the pie to a cooling rack and let it sit at room temperature for at least 3 hours. This allows the filling to set fully for clean slices.