In a food processor, combine all-purpose flour, kosher salt, and cold unsalted butter. Pulse until the mixture resembles pea-sized crumbs, about 10 seconds. Gradually add ice water, one tablespoon at a time, until the dough comes together. Divide into two discs, wrap in plastic, and chill for at least 1 hour.

Preheat your oven to 450°F (232°C).

Wash and chop the fresh rhubarb stalks into half-inch pieces. In a large mixing bowl, combine granulated sugar and additional all-purpose flour. Reserve a quarter of this mixture for the bottom of your crust.

Roll out one chilled dough disc into a 12-inch circle and fit it into a 9-inch pie pan. Sprinkle the reserved mixture on the bottom, add the chopped rhubarb, and top with the remaining sugar mixture. Roll out the second dough disc, place it over the filling, seal the edges, and cut slits for steam to escape.

Brush the top crust with a beaten egg and sprinkle with decorating sugar. Bake for 15 minutes at 450°F, then reduce to 350°F (177°C) and bake for another 45 minutes until golden and bubbling.