To make for a slightly less sweet, and more equally tart curd, reduce the amount of sugar to ½ cup. This will make for an equal parts sweet and tart lemon curd.
Make sure to only use stainless steel or lined metal pots or utensils. If you don't know if your whisk is stainless steel, make sure to only use it briefly when whisking together the ingredients. Use a rubber spatula when stirring the pot to prevent any prolonged contact with unlined metal.