Preheat the oven to 110°C, fan 90°C, gas ¼. Line a baking sheet with baking parchment. Put the egg whites in a clean, grease-free bowl with a pinch of salt and whisk with an electric handwhisk at medium speed until they form stiff peaks (when you lift the whisks, the whites should keep their peaks without flopping).
Whisking at high speed, add 1 tbsp sugar, then slowly add the rest, a little at a time, whisking until glossy and thick. Carefully fold in the cornflour and the vinegar.
Using a tablespoon, make 4 equal piles of the meringue mixture on the baking sheet, then make a hollow nest in the middle of each. Cook in the oven for 1 hour until the meringues lift cleanly off the baking parchment and are hollow when tapped. Turn the oven off and leave the meringues inside to cool.
Beat together the mascarpone and whipping cream until thickened and creamy, then beat in the vanilla, lemon zest and half the icing sugar. Set aside.
Put three-quarters of the strawberries in a food processor with the lemon juice and remaining icing sugar and whiz to a sauce.
Put the meringues on plates and spoon the mascarpone cream into the middle. Top with the blueberries, raspberries and the remaining quartered strawberries. Finish by drizzling over the strawberry sauce.