Prepare the crust: Preheat your oven to 180°C (160°C fan). Crush the digestive biscuits into fine crumbs — I use a food processor for even texture, but a rolling pin and sealed bag work well. Mix the crumbs with the melted butter and caster sugar until the mixture feels like damp sand and holds together when squeezed. Press this firmly into a 23cm loose-bottomed tart tin, working up the sides evenly. Bake for 10 minutes until the crust is fragrant, lightly browned at the edges, and firm to the touch. Cool completely on a wire rack.

Make the filling: In a large bowl, whisk the egg yolks with the lime zest until the mixture turns pale and thickens slightly, about 2 minutes. Add the sweetened condensed milk and whisk until smooth and glossy. Pour in the Key lime juice slowly while whisking — the mixture will thicken immediately as the acid reacts with the milk. The filling should be thick, creamy, and pale yellow with a silky sheen. Pour it into the cooled crust and spread evenly.

Bake the filling: Place the pie on a baking tray and bake at 180°C (160°C fan) for 12 minutes. The edges should be set and slightly puffed, but the centre will still jiggle like a soft custard when you gently shake the tray. Remove and cool on a wire rack for 30 minutes, then refrigerate for at least 4 hours — the filling firms up into a dense, sliceable texture.

Prepare the meringue: In a perfectly clean, dry bowl, beat the egg whites with the cream of tartar and a pinch of salt on medium speed until soft peaks form — the peaks should flop over when you lift the whisk. Gradually add the caster sugar, one tablespoon at a time, while beating on high speed. Continue until the meringue is stiff, glossy, and holds a firm peak that does not droop. The sugar should be fully dissolved — rub a little meringue between your fingers; it should feel smooth, not gritty.

Toast the meringue: Spoon the meringue over the chilled pie, starting at the edges and sealing them completely to prevent shrinking. Create dramatic peaks and swirls with the back of a spoon or a palette knife. Use a kitchen blowtorch for the most controlled toasting — move the flame in slow circles until the peaks are golden brown and caramelised in patches. Alternatively, place the pie under a hot grill for 1–2 minutes, watching constantly so it does not burn. The toasted meringue should smell sweet and warm, with dark brown tips against white clouds.

Garnish and serve: Let the pie rest at room temperature for 15 minutes after toasting so the meringue settles. Scatter extra lime zest over the top and arrange thin lime slices around the edge of the pie for a professional finish. Slice with a sharp knife dipped in hot water and wiped dry between cuts for clean edges.