Prepare the Pie Crust: In a large bowl, whisk the flour and salt. Cut in the cold, cubed butter using a pastry blender or your fingers until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Gradually add ice water, 1 tablespoon at a time, mixing until the dough just comes together. Divide the dough in half, form two discs, wrap in plastic wrap, and chill for at least 30 minutes.

Prepare the Fruit Filling: In a medium bowl, toss the chopped rhubarb and strawberries with 0.75 cup of sugar and 1 tablespoon of cornstarch until evenly coated. Let sit while you prepare the crust.

Assemble the Bottom Crust: Preheat oven to 375°F (190°C). On a lightly floured surface, roll out one dough disc to a 12-inch circle. Carefully transfer to a 9-inch pie plate. Trim the edges, leaving a 1-inch overhang, and chill while preparing the custard.

Prepare the Custard and Fill: In a separate bowl, whisk together the 3 eggs, 0.33 cup sugar, heavy cream, vanilla extract, and a pinch of salt until smooth. Gently fold in the fruit mixture, taking care not to crush the berries.

Bake the Pie: Pour the custard and fruit mixture into the chilled bottom crust. Roll out the top crust disc and place it over the fruit. Crimp the edges to seal, and cut several slits in the top crust to vent steam. Place the pie on a baking sheet to catch any drips.

Bake for 20 minutes at 375°F (190°C). Reduce the oven temperature to 350°F (175°C) and continue baking for another 35-40 minutes, or until the crust is golden brown and the center custard is mostly set (a slight jiggle is okay). If the edges brown too quickly, cover them loosely with foil.

Cool: Let the pie cool completely on a wire rack for at least 3-4 hours before slicing. This allows the custard to fully set up for clean slices.