Prep the oven and crust: Heat the oven to 400°F (200°C). Place a rack in the lower third. Set the chilled pie shell on a parchment-lined baking sheet.

Make the filling: In a large bowl, combine rhubarb, strawberries, sugar, cornstarch, lemon juice, vanilla, and salt. Gently toss until the fruit is evenly coated. Let it sit 10 minutes so the cornstarch hydrates.

Make the crumb topping: In a medium bowl, whisk flour, brown sugar, cinnamon, and salt. Cut in the cold butter with a pastry cutter or your fingers until clumps form and the mixture resembles coarse crumbs with pea-sized bits.

Assemble: Stir the filling again, then pour it into the pie shell, spreading evenly. Sprinkle the crumb topping over the fruit, covering it fully. Add coarse sugar on top if using.

Bake hot, then lower: Bake at 400°F for 20 minutes. Without opening the oven too long, reduce the temperature to 350°F (175°C) and continue baking 30–40 minutes, until the topping is deeply golden and the filling is bubbling thickly at the edges.

Shield if needed: If the crumbs or crust edge brown too fast, tent loosely with foil during the last 15–20 minutes.

Cool fully: Transfer the baked pie to a rack. Let it cool at least 3–4 hours to set the juices before slicing. For cleaner slices, cool to room temperature or chill briefly.

Serve: Enjoy plain, or with vanilla ice cream or lightly sweetened whipped cream.