1/2cupsugarif you use a sweeter citrus like orange, you'll want to decrease the sugar slightly
Combine juice, sugar, and eggs in a blender/food processor and blend for about 30 seconds. With the blades running, carefully pour in the melted butter.
Pour the mixture into a saucepan and bring to a low boil over medium heat (this may take a few minutes), stirring frequently and making sure to scrape the bottoms and sides. As soon as it reaches a boil, lower the heat and stir constantly until the mixture resembles pudding (it should coat the back of your spatula/spoon) and is a little bit shiny. This should only take a couple of minutes.
Pour into a jar and let cool before serving or storing.