Blind-bake the pastry: Preheat your oven to 190°C (170°C fan). Roll out the pastry to line a 23cm loose-bottomed tart tin, prick the base with a fork, line with parchment and fill with baking beans. Bake for 12 minutes until the edges look dry and pale golden. Remove the beans and parchment, then bake another 5 minutes — the base should feel sandy to the touch and smell toasty. Set aside to cool on a wire rack.

Mix the dry filling base: In a medium saucepan, whisk together the 200g caster sugar and cornflour until no lumps remain. This dry mix should look like fine white sand with no streaks.

Add the wet ingredients: Pour in the cold water and lemon juice, add the lemon zest, and whisk continuously. The mixture will look cloudy and thin at this stage, like pale milky water.

Cook the curd: Place the saucepan over medium heat, stirring constantly with a wooden spoon or whisk. After about 4 minutes, you'll see the mixture begin to steam at the edges. Continue stirring — it will suddenly thicken, turning from a thin liquid into a glossy, pudding-like texture that coats the back of your spoon. This takes roughly 6-8 minutes total. When you lift the spoon, the curd should hold a visible trail for a second before sinking back.

Finish the filling: Remove the pan from the heat immediately. Drop in the cubed butter and whisk until it melts completely — the curd will become even glossier and smell intensely of lemon. Add the egg yolks one at a time, whisking swiftly after each addition. The colour will shift from pale yellow to a deeper, warm golden hue. Pour the hot filling directly into the cooled pastry crust, spreading it evenly with a spatula. The surface should look smooth and shiny, like a lemon mirror.

Chill the pie base: Place the filled pie on a tray and transfer to the freezer for 15 minutes, or refrigerate for at least 2 hours, until the filling is completely set and firm to the touch — it should jiggle slightly but not ripple when you tap the tin.

Make the Swiss meringue: In a heatproof bowl set over a pan of barely simmering water (the bowl should not touch the water), combine the 150g caster sugar, egg whites, and cream of tartar. Whisk gently until the sugar dissolves — test by rubbing a tiny bit between your fingers; it should feel completely smooth, not grainy. This takes about 3-4 minutes. The mixture will look frothy and opaque, like a thin cloud.

Whip the meringue: Transfer the warm egg white mixture to a stand mixer fitted with the whisk attachment (or use hand beaters). Beat on medium-high speed for 5-7 minutes. The meringue will grow glossy, white, and voluminous, tripling in size. When you lift the whisk, the peak should stand up firmly without drooping. Add the vanilla extract and beat for 30 seconds more — the meringue should feel cool to the touch and look like satiny marshmallow fluff.

Assemble and brown: Spoon the meringue onto the chilled lemon filling, starting at the edges and sealing them completely against the pastry (this prevents weeping). Create peaks with the back of a spoon — they should hold their shape instantly. Use a kitchen blowtorch to brown the meringue, moving in steady circles until the tips are golden brown and toasted. Alternatively, bake at 200°C (180°C fan) for 3-4 minutes, watching closely — the peaks should become caramel-coloured and smell like toasted sugar.

Rest and serve: Let the pie sit at room temperature for 20 minutes before slicing. This rest allows the meringue to settle and the filling to soften slightly, giving a clean cut. The slice should show a crisp pastry base, a thick, jewel-bright lemon layer, and a cloud-like meringue top with browned peaks.