Step 1: Make the Pie Crust
In a large bowl, whisk together flour, sugar, and salt. Add cold cubed butter and use a pastry cutter or your fingers to work it into the flour until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining. Gradually add ice water, one tablespoon at a time, mixing gently until the dough just comes together. Divide dough in half, shape into disks, wrap in plastic wrap, and refrigerate for at least 1 hour.

Step 2: Prepare the Filling
In a large mixing bowl, combine sliced strawberries and rhubarb pieces. In a separate small bowl, whisk together sugar, cornstarch, lemon juice, vanilla, and salt. Pour this mixture over the fruit and gently toss until everything is evenly coated. Let the filling sit while you roll out the crust, allowing the flavors to meld.

Step 3: Roll Out Bottom Crust
Preheat your oven to 400°F (200°C). On a lightly floured surface, roll out one disk of dough into a 12-inch circle, about ⅛-inch thick. Carefully transfer it to your 9-inch pie dish, gently pressing it into the bottom and sides. Trim excess dough, leaving about a 1-inch overhang. Place in the refrigerator while you prepare the top crust.

Step 4: Assemble the Pie
Pour the strawberry rhubarb filling into the prepared bottom crust, spreading it evenly. Dot the filling with small pieces of cold butter. Roll out the second disk of dough into a 12-inch circle for the top crust (or create a lattice pattern if you prefer). Place the top crust over the filling, or weave your lattice. Trim and crimp the edges to seal. Cut 4-5 small vents in the top crust if using a solid top.

Step 5: Apply Egg Wash and Bake
Brush the top crust with beaten egg and sprinkle with coarse sugar if desired. Place the pie on a baking sheet (to catch drips) and bake for 25 minutes at 400°F. Reduce temperature to 350°F (175°C) and continue baking for 30-35 minutes more, until the crust is deep golden brown and the filling is bubbling through the vents. If the edges brown too quickly, cover them with foil or a pie shield.

Step 6: Cool Completely
Remove the pie from the oven and place on a cooling rack. This is crucial: let the pie cool for at least 4 hours, or preferably overnight, before slicing. This allows the filling to set properly so you get clean, beautiful slices instead of a soupy mess.