First, pre bake the crust. Position a rack in the center of the oven and preheat the oven to 375°F. Line a sheet pan with parchment paper and place the frozen crust in the center of the pan. Cover the crust with foil, making sure the crimped edges are completely covered and there are no gaps between the foil and the crust. Fill with pie weights and spread them out so they are more concentrated around the edges of the crust.
Pre bake the crust. Bake for 25 minutes. While the crust is in the oven, make the egg wash for the pie crust: in a small bowl, whisk together the egg, water, and salt. Prep the rhubarb for the filling by placing it in a medium bowl and tossing it in ¼ cup (1.75 ounces or 50 grams) granulated sugar.
Once the pie is done, remove the sheet pan from the oven, keeping the oven on. Carefully lift out the pie weights and the foil. Use a pastry brush to coat the bottom and sides of the pie crust (but not the crimped border) with a thin layer of egg wash. Bake, uncovered, for an additional 5 minutes. Cool on a wire rack while you make the custard filling.
Once the crust is cool, make the custard filling. Whisk together the remaining 1 ¼ cup (8.75 ounces or 248 grams) sugar, flour, eggs, butter and cream until combined. Pour the filling into the prepared crust and scatter the rhubarb evenly on top, discarding any liquid at the bottom of the bowl.
Bake the assembled rhubarb custard pie. Bake for 60 to 65 minutes, or until the edges of the filling are set but the center still wobbles slightly. Cool on a wire rack.
Serve and store. Serve warm or at room temperature. The pie can be stored at room temperature until ready to serve. After that, cover loosely with plastic wrap and refrigerate for up to 3 days.