Using a microplane or zester, zest one whole orange.
Separate egg yolks from the egg whites; Set both aside.
Mix flour, tapioca starch and sugar in a saucepan. Add orange zest and orange juice slowly, while stirring, over medium heat. Stir until thickened and boil for one minute.
Portion a half a cup of the boiling mixture and add to your egg yolks slowly, stirring very quickly with a whisk so your eggs don’t curdle. Add your yolk mixture back into the pot with the sugar mixture. Boil one more minute over medium heat, stirring constantly. It should be fairly thick and starting to cling to the whisk.
Stir in butter until melted and then pour into your pre-baked pie shell. Set aside.
Turn on oven to 375 degrees F. to preheat.
Meanwhile, beat egg whites and cream of tartar in the bowl of a stand mixer on high speed until frothy. Slowly add sugar until meringue is thick and glossy.
Spread meringue over pie and bake at 375 degrees for 10-12 minutes. Remove pie from oven and place pan on a cooling rack.
This pie is delicious warm, or chill several hours and serve cold.