Combine the flour, salt, and sugar in a large bowl. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with pea-sized pieces remaining. Drizzle in ice-cold water, 1 tablespoon at a time, mixing gently until the dough just holds together when pinched. Form into two discs, wrap in plastic, and refrigerate for at least 30 minutes.
Roll out one disc of dough on a lightly floured surface to about 12 inches in diameter. Transfer to a 9-inch pie dish, gently pressing into the bottom and sides. Trim excess dough, leaving a 1-inch overhang. Fold the overhang under itself and crimp decoratively. Refrigerate while preparing the filling.
Place the chopped rhubarb in a large colander and rinse thoroughly. Pat dry with paper towels to remove excess moisture.
In a medium bowl, whisk together the sugar, eggs, flour, and salt until well combined and slightly lightened in color. Stir in the heavy cream, vanilla extract, and nutmeg until smooth.
Preheat your oven to 425°F (220°C) with a baking sheet inside. Arrange the rhubarb pieces evenly in the chilled pie crust. Pour the custard mixture over the rhubarb and dot with butter pieces.
Place the pie on the preheated baking sheet and bake for 15 minutes. Reduce the oven temperature to 350°F (175°C) and continue baking for 30-35 minutes more, or until the center is just set with a slight wobble.
Allow the pie to cool for at least 2 hours before serving to ensure the custard sets properly.