1To 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 - it's OK if it's 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, the pastry can be frozen to use later on.
2When you're ready to bake, preheat your oven to 400F. On a lightly floured surface, roll the larger half of dough out into a 10-12-inch circle (depending on the size of your pie plate - you want it a bit bigger) and transfer to your 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.
3Scrape 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 help them connect, then trim the edge by running a knife around the edge of the plate, cutting away any excess pastry. Crimp the edge with your fingers and thumb, or a fork, or whatever you like or your grandma taught you.
4If you like, you can freeze your pie at this point. Otherwise bake it at 400 for 15 minutes, then turn the heat down to 350F for an hour, or until the crust is golden and the pie is bubbly. Let cool before slicing. Serves 8.