Place lemon juice in a small nonreactive bowl; sprinkle gelatin over.  Set aside.

Heat milk and 1/4 cup of the sugar in a medium saucepan over medium-low heat, stirring occasionally, until steaming and sugar is dissolved, about 5 minutes.  Meanwhile, whisk together yolks, 1T sugar, and cornstarch in a medium bowl until pale yellow and thickened, 3 to 5 minutes.  Whisking constantly, gradually add hot milk to the yolks.  Return milk and egg mixture to saucepan and cook, stirring constantly, over medium-low heat, until foam has dissipated to a thin layer and mixture thickens to the consistency of whipping cream and registers 185F, about 4 minutes.  Strain into medium bowl; stir in lemon juice mixture and zest.  Set bowl in large bowl of ice water; stir occasionally to cool.

While mixture chills, in the bowl of a standing mixer fitted with whisk attachment (or in a large mixing bowl if using a hand mixer), beat egg whites and cream of tartar on medium speed until foamy, about 1 minute.  Increase speed to medium-high; gradually add remaining 1T sugar and continue to beat until glossy and whites hold soft peaks when beater is lifted, about 2 minutes longer.  Do not overbeat.  Remove bowl from ice bath; gently whisk in about 1/3 of egg whites, then fold in remaining whites with large rubber spatula until almost no white streaks remain.

Beat cream on medium-high speed until soft peaks form when beater is lifted, 2 to 3 minutes.  Fold cream into custard and egg-white mixture until no white streaks remain.  Divide batter equally among four 3/4-cup ramekins (filled to the rim) or three 3/4-cup ramekins with foil collars.  Chill until set but not stiff, about 1.5 hours (can be refrigerated up to 6 hours); remove foil collars if using, and serve, garnished if desired.