Prep the crust: Preheat oven to 400°F. Press one crust into a 9-inch pie pan and refrigerate while you make the filling.
1 package pie crust
Mix the filling: In a large bowl, stir together rhubarb, strawberries, both sugars, cornstarch, salt, orange juice, and vanilla. Let sit while the oven heats—this draws out the juices.
3 ¼ cups rhubarb, 2 cups fresh strawberries, ⅓ cup light brown sugar, ⅓ cup granulated sugar, ¼ cup cornstarch, ¼ teaspoon salt, 1 tablespoon orange juice, ½ teaspoon vanilla extract
Add filling to pie: Use a slotted spoon to scoop the fruit (not the juice) into the crust. Dot the top with butter. Optional: sprinkle extra sugar if you like it sweeter.
2 tablespoons unsalted butter
Make the top crust: Cut strips from the second crust and create a lattice pattern, or simply place it on top with steam vents.
Seal and decorate: Crimp the edges and brush with egg wash. Sprinkle with coarse sugar if using.
1 large egg, Coarse sugar for garnish
Bake: Place pie on a rimmed baking sheet. Bake at 400°F for 20 minutes, then reduce to 350°F and bake 30–40 more minutes. Cover the edges if browning too fast.
Cool completely: Let cool on a rack for at least 3 hours. For best results, chill in the fridge another 3–4 hours (or overnight!) before slicing.