Preheat the oven to 150°C/130°fan/ gas 2. Line a 36cm x 25cm roulade tin (or shallow baking tray) with baking paper, leaving a 3cm overhang.
Whisk the egg whites in a clean bowl until very stiff, then gradually whisk in the lavender sugar and caster sugar until smooth and glossy. Whisk in the cornflour
Spoon the meringue mixture into the prepared tin and spread out evenly with a palette knife. Bake for 45 minutes, then leave to cool in the tin. Dust a large sheet of baking paper with icing sugar and turn out the meringue onto it. Carefully peel off the baking paper and leave to cool.
Spread the lemon curd over the cool meringue, then cover evenly with whipped cream. Roll up the meringue from the long side (using the baking paper underneath to help), then place on a serving plate, join-side down. Chill for 30 minutes.
Put the caster sugar and lemongrass in a pan with a splash of water and heat gently, stirring until the sugar is dissolved. Add the blueberries, then simmer gently for 5 minutes until they soften but still hold their shape. Leave to cool, then chill. Serve with slices of the roulade.