Step 1: In a large bowl, whisk together flour and salt. Cut in chilled butter using a pastry blender until pea-sized crumbs form. Gradually add ice water, one tablespoon at a time, until the dough comes together. Divide into two discs, wrap in plastic, and chill for 1 hour.
Step 2: Preheat your oven to 400°F (200°C) and place a baking sheet on the center rack.
Step 3: In a separate bowl, toss rhubarb, blueberries, sugar, cornstarch, lemon juice, zest, cinnamon, and a pinch of salt. Let sit for 15 minutes.
Step 4: Roll out one disc of dough on a floured surface to a 12-inch circle. Place into a 9-inch pie plate. Pour the fruit filling into the crust.
Step 5: Roll out the second disc and cut into strips to create a lattice top. Trim and crimp the edges to seal.
Step 6: Brush the top with a beaten egg and sprinkle with sanding sugar.
Step 7: Bake at 400°F for 20 minutes. Reduce heat to 375°F and bake for 35-40 minutes more until the filling is bubbling and the crust is deep golden brown.
Step 8: Allow the pie to cool completely on a wire rack for at least 3-4 hours before slicing.