Drain plums, dab dry with kitchen paper and cut into slices. Separate eggs. Whip sugar and egg yolk for about 5 minutes until thick and creamy. Add quark, sour cream, semolina and liqueur and mix well.
Beat the egg white and 1 pinch of salt until stiff and fold in. Grease an ovenproof casserole dish (1.1 l/ 20 x 30 cm; 3 cm high) well and sprinkle with semolina. Add the curd mixture and smooth it down.
Spread plum wedges on top. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 45 minutes. Take out and let cool down. Serve dusted with icing sugar.