In a large bowl, whisk together all-purpose flour and salt until well combined. Cut in cold butter using a pastry cutter or your fingers until the mixture resembles small peas. Combine cider vinegar and ice-cold water, then gradually add to the flour mixture. Mix until the dough holds together, then divide into two disks, wrap in plastic, and refrigerate for at least 45 minutes to allow it to chill and firm up.

Preheat your oven to 400°F. While the oven heats, wash and slice fresh strawberries and rhubarb into bite-sized pieces. In a mixing bowl, combine the sliced fruit with fresh lemon juice and red food coloring, if desired. In another bowl, whisk together granulated sugar, cornstarch, and a pinch of salt, then sprinkle this mixture over the fruit. Gently toss to thoroughly coat the strawberries and rhubarb.

Once your pie crust has chilled, roll out one disk on a lightly floured surface until it is about 1/8 inch thick. Transfer the rolled crust to a 9-inch pie plate, pressing gently into the edges. Carefully fill the crust with the prepared fruit mixture, dotting it with small pieces of butter for added richness. Roll out the second dough disk and cover the filled pie, crimping the edges together to seal. Cut several vent slits into the top crust.

Place your assembled pie on a baking sheet to catch any drips, then bake for 15 minutes at 400°F before reducing the temperature to 375°F. Continue baking for an additional 60 to 75 minutes, or until the filling is bubbly and the crust is golden brown.

After baking, remove the pie from the oven and let it cool on a wire rack for at least 2 hours to allow the filling to set properly. Serve it warm or chilled, and enjoy!