Preheat your oven to 425°F (220°C) and position a rack in the lower third of the oven. If you have a baking stone, place it on the rack to help ensure your bottom crust bakes properly.
Prepare your fruit by washing thoroughly. Hull and slice strawberries into 1/2-inch pieces. Trim the ends of the rhubarb and cut into 1/2-inch pieces, ensuring you’ve removed all leaves as they’re toxic.
Mix the filling by combining strawberries, rhubarb, sugar, tapioca, salt, lemon juice, vanilla, and cinnamon in a large bowl. Gently toss until all pieces are coated. Let the mixture sit for 15 minutes to allow the tapioca to begin absorbing juices.
Roll out your bottom crust to about 12 inches in diameter and transfer it to your pie dish, gently pressing it into the corners without stretching the dough. Leave the excess hanging over the edges.
Pour the fruit filling into the pie shell, making sure to include all the accumulated juices. Dot the top of the filling with the small pieces of butter, which will melt during baking and add richness.
Roll out the top crust and either place it whole over the filling (cutting vents) or create a lattice pattern by cutting strips and weaving them. Trim excess dough, leaving about 1/2 inch overhang.
Seal and crimp the edges by folding the overhang under itself and pressing to seal, then creating a decorative edge by pinching or using a fork to press around the perimeter.
Brush the top crust with beaten egg and sprinkle with the remaining tablespoon of sugar for a beautiful, golden, sparkly finish.
Place the pie on a baking sheet to catch any drips and bake at 425°F for 15 minutes, then reduce temperature to 350°F (175°C) and continue baking for 40-45 minutes until the crust is golden and the filling is bubbling.
Allow the pie to cool completely (at least 3 hours) before slicing to give the filling time to set properly. Patience here makes the difference between a slice that holds together and a runny mess!