In a large bowl, combine flour, sugar, and salt. Cut in cold butter until mixture resembles coarse crumbs.

Add ice water, one tablespoon at a time, mixing until dough comes together. Divide into two disks, wrap, and chill for 1 hour.

In a large bowl, mix rhubarb, strawberries, sugar, cornstarch, lemon juice, and cinnamon until well combined.

Preheat oven to 425°F (220°C). Roll out one dough disk and fit into a 9-inch pie dish.

Pour the filling into the crust. Roll out the second dough disk and create a lattice top or cover entirely, sealing the edges.

Bake for 15 minutes at 425°F, then reduce to 375°F (190°C) and bake for 45-50 minutes, until the crust is golden and filling is bubbly.

Cool for at least 3-4 hours before serving to allow the filling to set.