Combine all ingredients in a saucepan – I used a 2-quart – and set it over medium-low heat. Whisk until the butter is melted, and then continue to whisk constantly (or almost constantly; you don’t want to let it curdle or scorch) until the mixture is thickened, like jelly. As it thickens, you’re looking for it to hold the mark of the whisk, and if you lift the whisk, the mixture should hold its shape when it plops back into the pot. This will probably take about 10 to 12 minutes. When it’s ready, remove the curd from the heat, and press through a mesh strainer into a storage container. Chill well before serving.
Note: If you want your finished lime curd to still be flecked with bits of green lime zest, you could skip the straining step. But I always worry about finding bits of cooked egg in my fruit curd, so I like to strain mine – and then I lose the zest, but oh well.