Whisk together the sugar and egg until smooth in a microwave safe bowl. Gradually stir in the lemon juice, butter, and zest.Whisk all the ingredients until completely smooth and sauce like.Cook the mixture in the microwave for one minute, then in 45 second intervals. Make sure to stir well after each cooking period, remembering to scrape the sides well.Total cooking time should vary but shouldnt be more than 3-4 cycles. The lemon curd is ready when it is thick enough to stick to the back of a spoon.Transfer the curd to a mason jar. Keep in mind the curd will thicken as it cools.