In a large bowl, whisk together flour, salt, and sugar. Using a pastry cutter or your fingertips, work the cold butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces remaining. Add ice water one tablespoon at a time, stirring with a fork until the dough just comes together. Form into a disk, wrap in plastic, and refrigerate for at least 30 minutes.
Wash the rhubarb stalks thoroughly and trim off any leaves (they're toxic!) and tough ends. Cut into ½-inch pieces. If using particularly tart rhubarb, you can toss the pieces with 2 tablespoons of sugar and let sit for 15 minutes to draw out excess moisture.
Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out the dough to a 12-inch circle about ⅛-inch thick. Transfer to a 9-inch pie plate, trim the edges leaving a 1-inch overhang, then fold under and crimp decoratively. Refrigerate while preparing the filling.
In a medium bowl, whisk together the sugar, eggs, flour, salt, and nutmeg until smooth. Gradually whisk in the heavy cream, vanilla extract, and melted butter until fully combined. The custard should be silky and pourable.
Arrange the rhubarb pieces evenly in the chilled pie crust. Slowly pour the custard mixture over the rhubarb. The rhubarb will float—that's perfectly normal!
Bake for 15 minutes at 375°F, then reduce the temperature to 350°F and continue baking for 35-40 minutes, or until the custard is set but still slightly jiggly in the center. If the crust edges brown too quickly, cover them with foil or a pie shield.
Allow the pie to cool completely for at least 2 hours before slicing and serving. The custard will continue to set as it cools.