In a medium heatproof bowl, place eggs, sugar, lime zest, lime juice, and heavy cream, if using, & whisk to combine. Place the bowl over a saucepan of simmering water (bain-marie). Cook on moderate heat, whisking constantly, until mixture becomes thick (mine was ready in 10 minutes, but it can take up to 20). It should coat the back of a wooden spoon and leave a clear pass if you run your finger through it. The filling will thicken more once cooled.