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Melt the white chocolate in a heatproof bowl over a pan of simmering water, then remove from the heat. Whisk the egg whites to stiff peaks, add the sugar and briefly whisk again. Whisk the cream to soft peaks, then fold the melted chocolate into the cream. Once incorporated, mix a spoonful of the egg whites into the cream and chocolate mix, stir well, then fold the remaining egg white mix in. Work quickly, taking care not to knock out too much air. Cover with cling film and chill in the fridge for at least 1 hr.
While the mousse is setting, poach the rhubarb. Pour the wine, sugar, vanilla pod and seeds, and 150ml water into a saucepan and bring to the boil. Drop in the rhubarb, turn down the heat and poach for 2-3 mins. Turn off the heat and leave the rhubarb to cool in the liquid.
Heat oven to 180C/160C fan/gas 3. Mix the oats, flour and sugar in a bowl and pour over the melted butter. Stir well, then tip onto a non-stick baking sheet and bake for 20 mins, tossing halfway through, until golden and crisp. Remove from the oven and leave to cool. To serve, lift the rhubarb out of the poaching liquid and spoon into serving bowls, alternating with a large scoop of the mousse and a sprinkle of the oat crumb.