Heat oven to 160°C (140°C fan). Using a 20cm plate as a template, draw a circle on a large sheet of baking paper. Stick to tray with Spray oil.
In a well-cleaned, grease-free bowl, whisk the egg whites until they form stiff peaks. Add the caster sugar, 1 tbsp. at a time, continuing to whisk as you go. The meringue will start to get thick and glossy. Add the brown sugar in the same way, continue whisking for another 1-2 mins until really thick and holding stiff, shiny peaks on the end of your whisk. Now add the cornflour and vinegar, and give a final whisk until smooth.
Using a large metal spoon, pile the meringue onto the baking paper tray, using the circle template as a guide to create a symmetrical circle. Use the spoon to make a dip in the middle (this will hold your cream and fruit later). Place in the centre of the oven, shut the door and immediately turn the temperature down to 140°C (120°C fan). Cook the meringue for 1 hr 10 mins, then turn the oven off and leave until completely cold (best done overnight).
For the topping, put the 100ml honey in a wide frying pan and heat. Add the nectarine slices and cook for 1-2 mins on each side, over quite a high heat, until they are sticky but still holding their shape. Cool completely.
Just before you are ready to serve, assemble the Pavlova. Tip the cream, vanilla and remaining honey into a bowl. Whisk until it just holds soft peaks, then add the yogurt and, using a spatula, fold together until combined. Spoon the cream into the centre of the meringue and swirl it out towards the edges. Top with the sticky peaches and any saucy bits from the pan, and sprinkle over the pistachios.