1. Preheat the oven to 100C (80C fan-forced). Place the egg whites in the bowl of a freestanding mixer. Start to whisk using the balloon whisk attachment until the whites are light,frothy and reaching soft peaks. Keep the whisk going and start adding the sugar two tablespoons at a time until fully incorporated. Once it is thick,glossy and you have firm peaks,stir in the cornflour and vinegar.
2. Line a large baking tray with silicone-coated baking paper. Draw a 15-centimetre circle in the centre of the paper as a guide. Using two large serving spoons,scoop quenelles of the meringue into large dollops in a circle each just touching the next,leaving about a 15-diameter space empty in the middle. Repeat the quenelles in a circle all the way around to make a wreath shape until all the meringue quenelles meet. You should make about 10 quenelles.
3. Bake in the middle shelf of the oven for 1 hour 30 minutes,or until the outside and base is nice and crisp enough to move,and the inside is should still be soft and mallowy.
4. If you can,turn off the oven and leave the pavlova to cool inside. Otherwise,carefully transfer to a wire rack to cool.
5. To serve,place the wreath on a large,flat serving plate. Beat the yoghurt and dollop evenly over around the wreath. Scatter the pomegranate seeds and berries over the top and dust with the icing sugar. Add the pistachios,if using,and serve straight away.
Preparation time 30 minutes
Cooking time approx. 1 hour 30 minutes