Heat oven to 180C/160C fan/gas 4. Butter, then line a 23 x 32cm baking tray. Using an electric hand whisk, beat the egg whites, lemon juice and a pinch of salt until stiff.
Add 1 tbsp of the caster sugar, then whisk until the mixture makes stiff peaks. Repeat with the rest of the sugar, spoon by spoon, until the mix is thick and shiny. Spread into the tin, then bake for 15 mins or until crisp to the touch and lightly golden in places. Leave to cool.
Simmer the cherries, orange juice, sugar and cloves for 10 mins, or until the cherries are soft but still holding their shape. Mix the cornflour with 2-3 tsp cold water to make a paste, stir into the cherries and cook for a couple mins more until the juices thicken. Leave to cool.
To assemble, sift icing sugar over a large piece of parchment, then carefully flip the meringue onto it. Remove the tin and parchment. With a short end facing you, score a line 2cm into the meringue.
Whip the cream, icing sugar and vanilla until thick but not stiff, then spread over the meringue. Spoon over about half of the cherry sauce and scatter with the grated chocolate. Roll up the roulade, starting with the scored short end and using the paper underneath to help. Freeze the roulade for up to a month then defrost overnight. Freeze by setting on a parchment-lined tray then, once firm, wrap the parchment around it and cover with cling film. Freeze the remaining cherry sauce for up to a month.
To serve, drizzle with the melted white chocolate. Top with a few of the whole cherries and serve the remaining sauce on the side.