Juice the oranges and simmer the juice in a small pot over medium high heat until reduced by ½. This should take about 5-6 minutes, depending on your stove and pot.
Add the lemon juice and the zest, and cool the mixture.
Beat the egg and yolks together, then add the cooled juice mixture to the egg mixture in a slow stream, whisking constantly.
Strain the mixture back into the pot through a fine mesh strainer. Press the mixture through the mesh with the back of a spoon, extracting as much as possible.
Cook the mixture over medium-low heat, stirring constantly, until thickened. This should take 3-5 minutes.
Stir in the butter, mixing until completely incorporated.
Pour the curd into a jar (you'll get about 1 cup of curd), and store in the fridge.
Recipe by The Bright Kitchen at https://www.thebrightkitchen.com/citrus-curd/