Make the biscuit base: Pop the digestive biscuits into a sealed bag and bash them with a rolling pin until you've got fine crumbs — you'll want a sandy texture with no large chunks. Mix the crumbs with the melted butter until the mixture looks like wet sand and holds together when you pinch it. Press firmly into a 20cm tart tin, working up the sides. Bake at 160°C (fan) for 10 minutes until the base is fragrant and just starting to colour at the edges. Let it cool while you make the filling.

Whisk the filling: In a large bowl, beat the egg yolks and condensed milk together until smooth and pale yellow — you'll see the mixture thicken slightly as you stir. Add the lemon juice, a little at a time, and watch the mixture transform into a thick, creamy curd. Stir through the lemon zest. The filling should be glossy and smell intensely of fresh citrus.

Pour and set: Pour the lemon filling into the cooled biscuit base. It should spread evenly and look like a thick, yellow custard. Bake at 160°C (fan) for 15 minutes, until the filling is just set around the edges but still has a slight wobble in the centre. Remove and let it cool for 10 minutes.

Whip the meringue: In a clean, grease-free bowl, whisk the egg whites with the cream of tartar until soft peaks form — the mixture will look like a cloud of white foam. Gradually add the sugar, one tablespoon at a time, whisking continuously. Keep going until the meringue is stiff, glossy, and stands up in firm peaks. When you rub a little between your fingers, you shouldn't feel any graininess from the sugar. Fold in the cornflour gently with a spatula.

Top and swirl: Spoon the meringue over the warm lemon filling, spreading it right to the edges to seal the filling completely — this prevents weeping. Use the back of a spoon to create peaks and swirls across the surface. The contrast of the white meringue against the pale yellow filling is your cue that you're ready to bake.

Bake and cool: Bake at 180°C (fan) for 10–12 minutes, until the meringue peaks are golden brown and the valleys are just starting to colour. You'll smell a toasty, sweet aroma when it's ready. Let the pie cool completely on a wire rack — at least 2 hours — before slicing. The filling will be firm and the meringue will hold its shape perfectly.