In a microwave-safe bowl, whisk together sugar and eggs until smooth. Stir in lemon juice, lemon zest and melted butter.
Microwave on high in 1-minute intervals, stirring after each minute, until mixture is smooth, slightly thickened, and coats the back of a metal spoon. (All microwaves vary, but it took mine about 5 intervals / 5 minutes to reach desired consistency – smooth but still viscous. Curd will thicken as it cools.)
Pour curd into small sterile jars. Store for up to 3 weeks in your refrigerator.