Mix the lemon base: In a large bowl, whisk together the sweetened condensed milk, lemon juice, lemon zest (if using), and a pinch of salt until completely smooth and slightly thickened. The acidity of the lemon will start to thicken the mixture almost immediately.
Lighten with cream: In a separate small bowl, whisk the cold heavy cream by hand until it’s just slightly thickened and loosely mounded—it should not be fully whipped, just a bit aerated. Gently fold this cream into the lemon mixture until no streaks remain; this keeps the filling creamy and light while still setting firmly.
Fill the crust: Pour the lemon filling into the prepared graham cracker crust. Use a spatula to smooth the top so it’s level and glossy. If a few fine crumbs from the crust scatter onto the edge, just gently brush them toward the sides; a slightly rustic look is part of the charm.
Chill to set: Carefully transfer the pie to the refrigerator, uncovered, and chill for at least 3 hours, or until the filling is fully set and the top looks smooth and firm to the touch. For the cleanest slices, chill 4 hours or longer.
Slice and serve: When ready to serve, run a thin knife around the edge of the crust to loosen it slightly from the glass. Slice the pie with a sharp knife, wiping the blade between cuts for neat wedges. Serve cold, optionally topped with a little whipped cream and a few fresh berries.
