Pour the orange juice into a small sauce pan over medium-high heat. Bring the juice to a rapid simmer and let it reduce down to approximately 1/4 cup. This should take 2-4 minutes.
Transfer the orange juice to a measuring cup to cool. Stir in the zest and lemon juice.
In a separate bowl, whisk together the egg, yolks, and sugar. When the orange juice has cooled to room temperature, whisk it into the egg mixture in a steady stream.
Place a clean bowl with a strainer over the top next to the stove.
Pour the egg and juice mixture back into your small sauce pan and set it over medium heat. Stir slowly but constantly until the mixture has thickened to a pudding-like consistency - about 6-8 minutes or until the mixture is 180°.
Strain the mixture into the clean bowl to remove the zest and any bits of cooked egg. (Alternatively, you can leave the zest in the curd for a chunkier texture.) Stir in the butter and the pinch of salt while the curd is still warm.
Store the curd in a clean jar with a lid. It will keep refrigerated for about a week.