Make and bake the base: Heat the oven to 325F. Blitz the biscuits to fine crumbs, toss with the melted butter until every crumb looks like damp sand, and press firmly over the bottom of a 9-inch springform pan with the back of a measuring cup. Bake 10 minutes, brush the hot base with the beaten egg white, and return to the oven for 2 minutes to seal. Cool completely.
Beat the filling gently: Drop the oven to 300F. Beat the cream cheese until completely smooth, then beat in the sugar, cornstarch and salt. Add the eggs and yolk one at a time on low speed, scraping the bowl after each - do not whip in air. Stir in the sour cream, lemon zest, lemon juice and vanilla until just combined.
Bake low and slow: Pour the filling over the cooled base and smooth the top. Bake 50 to 55 minutes, until the edges are set but the center still has a slight wobble when you nudge the pan.
Cool it the Ramsay way: Turn the oven off, prop the door open with a wooden spoon, and leave the cheesecake inside for 1 hour. Then refrigerate overnight, at least 8 hours, to set fully. Run a thin knife around the edge before releasing the springform.
Whip the meringue fresh: Just before serving, whisk the egg whites with the cream of tartar to soft peaks. Add the warm sugar a spoonful at a time, whisking to stiff, glossy peaks that hold their shape.
Pile, swirl and torch: Spoon the meringue onto the cold cheesecake and swirl into tall peaks with the back of a spoon. Torch until golden, moving constantly, or broil 2 to 3 minutes watching closely. Slice with a hot, dry knife and serve immediately.
