07Increase the heat to medium and cook, stirring constantly, until the mixture thickens, about 15 minutes.
08The curd is cooked when it reaches 170°F, but you can see that it's cooked when your finger leaves a clear path on the back of a spoon. Don't let the mixture boil.
09Remove the curd from the heat; stir in the lemon zest.
10Transfer the curd to a bowl.
11Press plastic wrap on the surface of the lemon curd to keep a skin from forming and chill the curd in the refrigerator