In large microwave safe bowl, whisk sugar and eggs. Add melted butter, whisking to combine. Finally add lemon juice and option lemon rind, whisking until smooth. Put the bowl on a plate (to catch any bubble-overs), place in the microwave, and cook in 1-minute increments, stirring after each. Depending on the power of your microwave, it should take 8-10 minutes on High. Curd will just be thickening and will coat the back of a spoon. Serve warm over gingerbread or refrigerate to thicken.