For the crust: add the flour, sugar, and salt to a food processor and mix to combine. Then add the cold butter in pieces (cut each stick into about 5 slices), and pulse until the mixture is crumbly with pea-sized lumps. Drizzle the ice cold water in slowly as the processor is running -- stop when the dough starts to come together in a ball. You may only need 1/3 cup of water, add more as needed.
Turn the dough out onto a surface and gently push it together into 2 flattened discs. Cover each disc with plastic wrap and refrigerate or freeze until ready to use.
Make the filling: boil a medium pot of water. Once water is boiling, make an "x" with a knife at the base of your peaches and drop them in the pot. Let them boil for about 2-3 minutes, then take them out with a slotted spoon and drop them in a bowl of ice water until they cool down. Once they are cool enough to handle, peel them! The skin should slip right off if you start where you made the "x". Once peeled, slice them into 1/3 inch thick slices.
Add the sliced peaches, strawberries, lemon juice, sugar, and cornstarch to a medium bowl and mix gently to combine. **You can use cornstarch or flour or a combination to thicken the filling. I like using 90% cornstarch and 10% flour to make 1/4 cup.**
Preheat the oven to 375.
Roll out the two discs of your pie crust separately on a floured surface until about 1/4 inch thick. (But seriously, REALLY flour your counter! Pie dough is sticky business.) Transfer the first rolled out piece to your pie pan.
Fill the crust with your fruit filling and any juice. If your fruit happened to be extra ripe and very juicy, add a little extra flour to the filling, or just leave some of the juice out. Also you can always reserve the juice, boil it down in a small saucepan, and then add it to the filling so it is less liquid-y and more concentrated.
Now using a knife or a pizza cutter, slice your second rolled-out disc of crust into long strips, and lay them across your pie in a lattice pattern. Trim the excess edge crust and crimp it along the outside with your fingers, a fork, or a spoon.
Brush the top of your pie with a pastry brush dipped in egg white, then bake it for around 40-50 minutes (the top should be golden brown and the fruit should be bubbling).