Start by making the lemon curd. In a large, microwavable bowl, add the sugar, butter and the zest of both lemons but the juice from 1 1/2. Microwave on a medium heat until the butter has melted and the sugar has dissolved, mixing every 30 seconds. Once melted, add the flour and allow the mixture to cool slightly before cracking in the whisking in the egg and placing back in the microwave to cook until thick, mixing every 30 seconds. Place in the fridge to cool completely.