Preheat your oven to 425°F (220°C). In a medium saucepan, combine granulated sugar, cornstarch, salt, and freshly squeezed orange juice. Cook over medium heat, stirring continuously until the mixture thickens and becomes clear, about 4-5 minutes. In a separate bowl, beat egg yolks, then gradually mix in some of the hot mixture before returning all to the saucepan. Continue to cook for another 2-3 minutes, then remove from heat and stir in lemon juice, orange zest, and butter until smooth.

Pour the vibrant filling into your pre-baked pie crust.

In the bowl of a stand mixer, combine egg whites and cream of tartar. Start beating on medium speed until frothy and soft peaks begin to form, about 2-3 minutes. Gradually add granulated sugar and powdered sugar, continuing to beat until you achieve glossy, stiff peaks, typically around 3-5 minutes.

Spoon the meringue over the orange filling, ensuring it completely covers the surface. Use a spatula to create swirls or peaks in the meringue. Place in the oven and bake for 8-10 minutes until the meringue is golden brown.

After baking, allow the pie to cool at room temperature for at least one hour. Refrigerate until ready to serve. Cut into slices and serve chilled.