In a food processor, pulse flour, salt, and sugar until combined. Add cold butter, blending until resembling coarse crumbs. Gradually add ice water, pulsing until clumped. Shape into a disk, wrap, and chill for at least 1 hour.

Combine chopped rhubarb and strawberries in a bowl. Add sugar, cornstarch, lemon zest, and juice, stirring gently. Allow to macerate for 15 minutes.

Preheat oven to 400°F (200°C) and position rack in lower third.

Flour the countertop and roll dough into a rough 13-inch round on parchment paper.

Transfer the dough to a baking sheet. Spoon fruit mixture into the center, leaving 2 inches of dough. Fold edges over the filling.

Brush the crust with egg wash and sprinkle with turbinado sugar.

Bake for 40-45 minutes until golden-brown. Tent edges with foil if browning too quickly.

Cool for at least 30 minutes before slicing.