Wash the plums, drain, halve and stone them. Bring to the boil with 250 ml water and 4 tablespoons of sugar and cook over a low heat for about 8 minutes. Let it cool down a little and put it into a greased casserole dish (1 litre capacity). Heat 125 ml milk. Put the rusks on a baking tray and pour the warm milk over them. Carefully place the soaked onions on the compote with a spatula, close together. Mix pudding powder and 3 tablespoons of sugar.
Gradually stir with 6 tablespoons of cold milk until smooth. Bring the remaining milk to the boil, remove from the heat and stir in the pudding powder. Boil the pudding while stirring for 1-2 minutes and put it hot on the onions. Beat egg white and lemon juice until stiff, add 125 g sugar and beat for another 5 minutes. Add egg white with a spoon to the pudding. Bake the casserole on the lower shelf in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 12 minutes. Remove, allow to cool slightly and dust with icing sugar before serving