To candy the hibiscus, soak the flowers in 100ml of boiling water for five minutes. Remove (keeping the liquid) and pat dry. brush eight of the flowers with a thin, even coating of egg white. Place the sugar in a small bowl and toss them lightly, coating them evenly. Spread on a separate lined baking sheet, and cook them at the same time and for as long as the meringue or until dried but not browned.
To cook the meringue, preheat the oven to 140c. Line a large baking sheet with parchment paper. Heat the sugar and water in a pan until it reaches 115c. This should take about 5 minutes of boiling. If you don’t have a thermometer drop a little of the sugar syrup into a deep bowl of cold water. If it’s reached temperature it should form a soft ball. If you remove the ball from the water it will flatten after a few seconds.
Whisk the egg whites with the lemon juice in a very clean, preferably metal bowl or mixer until it stiffens. Then carefully pour in the hot sugar syrup, slowly, whilst whisking bit by bit until fully incorporated and very stiff. Either pour the meringue onto the parchment and spread out into a circle about 25-30cm wide and 3-5cm high or spoon 8 large individual meringues onto the parchment. Put in the oven for 2 hours above the hibiscus flowers, turn the oven off and leave until the oven cools. This can be done the day before if necessary.
To roast the rhubarb, preheat the oven to 180C. Spread the rhubarb on a baking tray, sprinkle with caster sugar, the vanilla essence and the hibiscus flower water. Cover with parchment and put into the oven for ten minutes. Remove parchment and check the rhubarb is cooked by gently pressing a piece. They should be just soft when they are cooked. If they aren’t quite cooked return to the oven without the parchment and keep cooking. When they are done, remove from the pan to prevent them from continuing to cook.
To make the rhubarb curd, place the rhubarb in a small saucepan with 100ml of water, bring to a simmer, place the lid on top and cook gently for 10 minutes or until the rhubarb is soft. Push the softened rhubarb through a sieve with the liquid and allow to cool.
Return the rhubarb puree to a saucepan and stir in the sugar and the egg yolks or whole eggs if using and place on a low to medium heat. Stir constantly, scraping the sides and bottom with a spatula until it thickens to coat the back of a wooden spoon or until it reaches a temperature of 75C. Remove from the heat and stir in the butter. Strain through a sieve pour into a sterilised jar, allow to cool, then refrigerate for up to a month. Any leftovers are delicious spread on toast.
To assemble the pavlova, If necessary whisk the cream until just stiff but not overly so. Good cream with a high fat content thickens very easily and can turn buttery if you are not careful so steer on the side of caution. Place the cream on top of the cold meringue alongside a blob of rhubarb curd then pour over the roasted rhubarb and syrup. Decorate with the candied hibiscus flowers.