In a large bowl, whisk together the heavy cream, sweetened condensed milk, lemon zest, and lemon juice. Pour into a bread loaf pan, cover with plastic wrap, and freeze for one hour.
In the metal bowl of a stand mixer, add the egg whites, sugar, and cream of tartar. Place the mixing bowl over a pan with an inch of simmering (not boiling) water and gently whisk until frothy, and all the sugar has dissolved. Remove from heat, then lock into stand mixer with the whisk attachment and whip for 1 minute on medium speed, then 1 minute on medium high, turn the speed to high and whip until the meringue holds glossy peaks and the bottom of the bowl is cool to the touch, around 10 minutes. Stir in the vanilla, then scoop into a plastic bag with one corner cut off.
Remove the ice cream from the freezer and gently stir. While stirring, slowly pipe the meringue into the ice cream, using as much or as little as desired, until swirling with white stripes. Smooth out the top of the ice cream, re-cover, and freeze for an additional 3-4 hours, or until the ice cream is firm.