Heat the whole milk and heavy cream in a saucepan over medium heat until steaming, about 3–5 minutes, with small bubbles forming around the edges (no boiling).

Whisk the egg yolks, granulated sugar, and cornstarch in a bowl until pale and thick, about 1 minute, so the mixture ribbons off the whisk.

Slowly whisk the hot milk mixture into the egg yolk mixture to temper it, keeping the flow steady to avoid scrambling (about 1 minute).

Return everything to the saucepan and cook over medium heat, stirring constantly, until thickened to a pudding consistency, about 5–8 minutes, with a glossy coat on the spoon.

Remove the custard from the heat and stir in the fresh lemon juice, lemon zest, vanilla extract, and salt until smooth and evenly bright, 30–60 seconds.

Cool completely over an ice bath with occasional stirring, about 10–15 minutes, until no longer warm to the touch.

Refrigerate at least 4 hours, covered, until very cold throughout for best texture.

Churn in an ice cream maker on the lowest setting until dense and thick, about 20–30 minutes, with a scoopable gelato texture.

Serve immediately for soft-set gelato, or freeze 1–2 hours to firm up so it holds a quenelle shape.