How to Make It
Heat oven to 375° F. In a large bowl, toss the rhubarb, strawberries, sugar, flour, orange zest, cinnamon, and salt. Lightly beat one of the eggs, add to the fruit, and toss to combine.
Fit one of the piecrusts into a 9-inch pie plate. Fill with the fruit mixture, pressing it in firmly. Lay the second crust on top and press the edges of the crusts together to seal.
Trim the crust to a 1-inch overhang, fold the overhang underneath itself, and crimp. In a small bowl, beat the remaining egg with 1 teaspoon water and brush over the crust. Sprinkle with sugar and cut vents in the top.
Place the pie on a foil-lined baking sheet and bake until the fruit mixture is bubbling and the crust is golden brown, 1¼ to 1½ hours. Let cool for at least 2 hours before serving.