Sweet and tart filling, buttery and flaky crust; strawberry rhubarb pie is on my must-make list every spring from now on.
Combine all ingredients except buttermilk in the bowl of a food processor. Pulse until butter is well combined with the flour and the mixture looks sandy. Add buttermilk and pulse a few times until a dough forms and starts to ball up in the bowl.
Turn the dough out onto a clean surface, divide in half, then shape each half into a flat disc. Wrap each disc in plastic and refrigerate for at least 30 minutes (and up to 5 days, if making ahead) while you prepare the filling.
Stir together the rhubarb, strawberries, sugar, cornstarch, and salt in a large bowl until fruit is thoroughly coated in the mixture. Set aside while the oven preheats and you roll out the bottom crust. Juices will accumulate at the bottom of the bowl during this time.
Preheat your oven to 400 degrees F.
Take one of the chilled discs of pie dough out of the fridge (leave the other one in) and roll it out on a lightly floured surface until it is 12 inches in diameter. Carefully place it in the bottom of a 9-inch pie dish, patting it down and making sure it is flush with the bottom and sides of the dish. There will be some overhang around the edge of the pie dish; that's ok.
Using a slotted spoon, scoop the filling into the crust, leaving the juices that have accumulated behind (if you don't do this, you may have soggy pie). Discard the juices.
Dot the cubed butter over the filling. Place the pie in the fridge while you roll out the second crust.
Remove the second disc of dough from the fridge and roll it out until it is 12 inches in diameter. Using a knife or pizza cutter, cut the dough into strips 1 inch wide.
Remove the pie from the fridge and arrange the strips in a lattice pattern, weaving them over and under each other. Once the lattice is complete, bring the excess bottom crust overhang up over the edges of the strips and crimp the pie crust with your fingers or a fork.
Whisk together the egg and milk or cream in a large bowl. Brush the mixture over the crust. Sprinkle with the coarse sugar, if desired.
Bake the pie for 20 minutes at 400 degrees, then turn the temperature down to 350 degrees and bake for 35-40 more minutes. I like to put a pie shield over the outer rim of the crust (aluminum foil works great) once I turn the temperature down, to prevent it from burning. Pie is done when the crust is golden brown and the filling is bubbling.
Here is the critical part: let the pie cool completely for AT LEAST 4 hours, and preferably overnight, before serving. This allows the filling to set. If you do not wait, your filling may be soupy. But still delicious. Serve with vanilla ice cream or whipped cream!
*If you don't have buttermilk, you can use 1/2 Tablespoon vinegar combined with 1/2 cup regular milk, or I also use buttermilk powder (Saco brand is in most stores). You just add the powder to the dry ingredients, then add cold water when the buttermilk is called for. Works great!
Pie will keep, covered in the fridge, for up to 5 days. My favorite way to serve leftover pie is to microwave each slice for 20-30 seconds or until hot and then serve with vanilla ice cream.
This strawberry rhubarb pie can also be frozen once baked, tightly wrapped and covered, and stored in the freezer for 2-3 months; thaw in the fridge overnight and then warm in the oven or microwave to serve.
Pie filling adapted from Sally's Baking Addiction.