Whisk together the egg and sugar in a large microwave-safe bowl. Then whisk in the melted butter and lemon juice until thoroughly combined. Be sure to use a bowl that is big enough to avoid bubble-overs. The lemon mixture should fill no more than 1/4 of the bowl.
Microwave the mixture for 1-minute increments, removing from the microwave and stirring to combine after each minute.
When the curd starts to thicken (sticks to the back of a spoon and starts to mound a bit as you stir) it's done. (It will be like a gel consistency, but will thicken more once cooled.) If you are unsure, you can look for a temperature of about 187°F/86.1°C. Cook time will take anywhere from 4 to 10 minutes, depending on the strength/power of your microwave.
Stir the curd one more time, spoon it into a storage container, and refrigerate until firm.
Keep curd refrigerated for up to 3 weeks; freeze for longer storage.
Avoid using a metal utensil to mix the hot lemon mixture, in order to avoid a metallic taste.