1. Preheat oven to 140°C, gas mark 1. On the back of a piece on non-stick baking parchment, draw a rectangle about 12x30cm, then line a flat baking sheet.
2. Place the egg whites in a really clean and grease-free bowl with the cream of tartar and whip with an electric whisk until they hold their shape and form soft peaks. Gradually add the sugar, continuing to whisk until the meringue is glossy and thick. Fold in the cornflour and the vinegar.
3. Using your drawn outline on the reverse as a guide (you should be able to see it through the parchment), spoon the meringue onto the baking sheet in a rectangle, flattening the base and running a palette knife around the edges for smooth, straight sides. Bake for 1 hour until crisp on top. Remove from the oven and cool slightly before carefully transferring onto a serving plate or board. Leave to cool completely before removing the paper from the base, as this will help prevent the meringue from cracking.
4. Whip the cream to soft peaks and pile into the centre. Cut any large strawberries into halves, quarters or slices, then scatter with the rest of the berries over the cream and serve.