Whip the creamUsing a hand mixer or a whisk, whip the cold heavy cream to soft peaks. Don't take it all the way to stiff: over-whipped cream folds in badly and costs you some of the smoothness. If your kitchen runs warm, chill the bowl first.
Mix the lemon and condensed milkIn a separate bowl, stir the sweetened condensed milk together with the lemon juice and the zest, if you're using it. The mixture will thicken almost immediately. That's the acid doing its job, and it's exactly what should happen.
Fold the two togetherAdd the lemon mixture to the whipped cream and fold it in with a spatula, using slow lifting strokes, until there are no streaks left. Take your time here: the more air you keep, the creamier the final texture.
FreezeTransfer to a lidded container and freeze for 5 to 6 hours, until it reaches the texture you like. At 5 hours it's soft and spoonable; at 6 it scoops cleanly.