Step 1: Prepare your pie crust. Roll out one half of the dough into a 12-inch circle on a lightly floured surface. Carefully transfer it to a 9-inch pie plate. Trim the edges, leaving about a 1/2-inch overhang. Place in the refrigerator while you prepare the filling.
Step 2: In a large bowl, gently combine the sliced rhubarb and hulled, sliced strawberries. Aim for uniform pieces to ensure even cooking.
Step 3: In a separate medium bowl, whisk together the granulated sugar, cornstarch (or flour, if preferred), lemon zest, and a pinch of salt. Cornstarch helps to thicken the juices released by the fruit, preventing a soggy pie.
Step 4: Pour the sugar mixture over the fruit and toss gently to coat thoroughly. Allow it to sit for 10-15 minutes; this helps draw out some of the fruit juices and dissolve the sugar.
Step 5: Pour the fruit filling into the chilled pie crust in the pie plate, mounding it slightly in the center. Dot the top with small pieces of butter for added richness and flavor.
Step 6: Roll out the second half of the pie dough. You can create a full top crust, cut out decorative shapes, or make a lattice top. For a lattice top, cut the dough into 1-inch wide strips and weave them over the filling.
Step 7: Trim and crimp the edges of the top and bottom crusts together to seal. Brush the top crust with an egg wash (one egg yolk beaten with a tablespoon of water) and sprinkle with a tablespoon of coarse sugar for a beautiful, golden, sparkling finish.
Step 8: Preheat your oven to 425°F (220°C). Place the pie on a baking sheet to catch any potential drips. Baking on a hot baking sheet can also help prevent a soggy bottom crust.
Step 9: Bake for 15-20 minutes at 425°F (220°C), then reduce the oven temperature to 375°F (190°C) and continue baking for another 40-50 minutes, or until the crust is golden brown and the filling is bubbly. If the crust edges start to brown too quickly, loosely cover them with aluminum foil. The fruit should be tender when pierced with a knife.
Step 10: Once baked, remove the pie from the oven and let it cool completely on a wire rack for at least 3-4 hours before slicing. This crucial step allows the filling to set properly. Cutting into a warm pie too soon will result in a runny filling.