Zest one of the oranges. Slice both oranges in half and juice them. Pour the juice into a small saucepan over medium-high heat. Bring to a rapid simmer and cook, stirring, to reduce to about 1/4 cup, 2-4 minutes. Transfer to a measuring cup to cool, and stir in the orange zest and the lemon zest. Set aside.
In a large bowl, whisk together the egg, egg yolks, and sugar. When the orange juice has cooled to room temperature, whisk it into the egg mixture in a slow, steady stream.
Set a fine-mesh strainer over a clean bowl, and set aside.
Transfer the egg and juice mixture back into the saucepan. Cook, gently and constantly stirring, until the mixture turns into a pudding-like consistency, 6-8 minutes (or until the curd reaches 180˚F.) Strain the curd through the strainer to remove any bits of cooked egg and zest. Stir in the butter and a pinch of salt.
Store in a covered container in the refrigerator. It will keep about a week.