To make the pastry, combine the flour, sugar and salt in a large bowl. Add (or grate in) the butter and blend with your fingers or a pastry cutter until the mixture is coarse, with pea-sized pieces of butter. If you like, rub them between your fingers and thumbs to make flat flaky pieces of butter. Add the water and stir just until the dough comes together—don’t worry if it looks a bit shaggy. Gather into a ball, divide in half (I usually make once piece slightly smaller than the other, for the lid), shape each into a disc, wrap in plastic and let rest or refrigerate for an hour (or up to 3 days). Alternatively, let it rest for about 20 minutes and use it right away, or freeze to use later on.
When you're ready to bake, preheat your oven to 400?F. On a lightly floured surface, roll the larger half of dough out into a 10-12-inch circle and transfer to the pie plate, gently pressing it into the bottom and letting the edges hang over. Put your rhubarb and strawberries into a large bowl (you can use the same one without needing to wash it) and in a small bowl or measuring cup, stir together the sugar and cornstarch. Pour over the fruit and toss to coat.
Scrape the fruit into the pie crust and roll out the lid in the same manner. Drape over the pie, pressing down around the edge to seal, trim away the excess pastry and crimp the edge if you like.
You can wrap and freeze your pie at this point, then bake it from frozen, adding an extra 15 minutes or so of baking time. Otherwise, bake it at 400?F for 15 minutes, then turn the heat down to 350?F for an hour, or until the crust is golden and the pie is bubbly. Let cool before slicing. Serves about 8.