Preheat the oven to 200C fan/220C/425F/gas mark 7.
Place the tarragon sprigs for the roast plums in the base of a ceramic dish or roasting tin with 150ml water. Lay the plums over these, cut side up, ideally in one layer. Sprinkle the sugar onto the plums, macerate for 15 minutes, then bake in the oven for 20–30 minutes, until soft and puffy but still plum shaped. Leave to cool for 15 minutes, then transfer the plums, juices and tarragon to a smaller vessel and refrigerate until required.
Weigh the egg whites. You will need double the quantity of sugar to whites to make a classic meringue, and ideally a stand mixer fitted with the balloon whisk to do the hard work. Set the oven to 130C fan/150C/300F/gas mark 2 and line a baking sheet with baking parchment or a silicone baking mat.
Ensure the mixing bowl is spotless. Add the egg whites and whisk at medium speed until they form stiff peaks. Increase the speed and sprinkle the sugar into the egg whites in a steady stream. Continue whisking for nine minutes, until the mixture is glossy and, if pinched, has no hint of sugary crystals.
Scatter 35g toasted almonds over the meringue mixture. Use a large metal spoon to carefully but confidently ripple through the nuts in just two or three large swoops. Spoon the mixture onto the lined baking sheet creating a circle 26–28cm in diameter, with high, wavy sides and an indent for cream and fruit in the middle. Place in the centre of the oven, reduce the oven temperature to 110C fan/130C/250F/ gas mark 1/2 and bake for two hours, then turn the oven off and leave for one hour more.
Remove the meringue from the oven and sprinkle a further 35g of almonds on top. Whip the cream and vanilla extract in a bowl until they reach ‘loose ribbon stage’, then spoon on top of the meringue. Arrange the plums and juices over the cream, then scatter with fresh tarragon and the remaining almonds.