In a large mixing bowl, combine sugar, flour, and nutmeg. Stir to mix evenly.
Rinse the rhubarb and pat dry. It does not need to be peeled. Dice into small ½-inch pieces. Measure out 3 cups of diced rhubarb and add to the sugar and flour mixture. Set aside.
Whisk together orange zest, vanilla extract, heavy cream, and eggs. Whisk to thoroughly beat the eggs into the mixture.
Pour the cream mixture into the rhubarb mixture and stir to combine.
Pour filling into the unbaked pie crust.
Dot small pieces of the butter over the top of the filled pie.
Bake for 45 to 50 minutes, covering the crust loosely with aluminum foil after the first 20 minutes once the crust has browned.
Remove from the oven and cool completely on a cooling rack.
Notes
Rhubarb should be cut into bite-sized pieces. I mix it with the flour and sugar to help it stay suspended in the custard filling.
The orange zest and nutmeg are optional but add nice complementary yet subtle flavors.
You can dust the top of the pie with powdered sugar just before serving if desired.