Heat the whole milk and heavy cream together in a saucepan until steaming, with small bubbles forming at the edges (about 5 minutes). Visual cue: the surface should look lightly steamy, not boiling.

Whisk the egg yolks, granulated sugar, and cornstarch until pale and thick (about 1 to 2 minutes). Visual cue: the mixture should look lighter in color and ribbon off the whisk.

Slowly whisk the hot milk mixture into the egg mixture in a steady stream, keeping the yolks from scrambling (about 1 to 2 minutes). Visual cue: the custard should turn smooth and glossy.

Return the mixture to the saucepan and cook over medium heat, stirring constantly, until thickened to a pudding consistency (about 5 to 7 minutes). Visual cue: it should coat the back of a spoon and leave a clear line when you run a finger through it.

Remove the saucepan from heat and stir in the fresh lemon juice, lemon zest, vanilla extract, and salt. Visual cue: the base should become vivid and intensely lemon-scented.

Cool completely over an ice bath, stirring occasionally, until the custard feels cool to the touch (about 10 to 15 minutes). Visual cue: the mixture should thicken slightly as it cools.

Refrigerate the custard for at least 4 hours. Visual cue: it should be very cold all the way through before churning.

Churn the chilled custard in an ice cream maker on the lowest setting for a dense texture. Visual cue: it should look like thick, pale-yellow gelato that holds shape.

Serve immediately for soft-set gelato, or freeze for 1 to 2 hours for a firmer scoop. Visual cue: after freezing, it should be dense and easy to form.