Wash, halve and stone the plums. Wash oranges hot, dab dry. Cut 6 slices from 1 orange. Caramelise 100 g sugar in a pot while stirring. Add fat, turn orange slices briefly in it and put them on oiled aluminium foil.
Dissolve caramel with 1/8 litre hot water, add cinnamon and bring to the boil. Steam plums for about 5 minutes in caramel stock. Set aside. For the zabaglione, grate the zest of the second orange, squeeze the juice.
Cream egg yolk, egg and remaining sugar in a metal bowl. Add 8 cl orange juice and the orange peel and beat until creamy on a hot water bath. Place in a cold water bath and stir until cold.
Drain the plums a little and arrange each with a slice of orange. Fill the zabaglione on top and sprinkle with cinnamon. Serve garnished with lemon balm.