Roast the almonds in a pan without fat until golden brown. Take them out and let them cool down. Remove the pistachios from their shells. Grind 30 g pistachios together with the almonds in the universal chopper, chop the rest coarsely.
Set aside. From the watermelon cut 2 slices in the middle, from the other melons cut 1 slice each (approx. 2.5 cm thick) in the middle. Use the rest of the melons for other purposes.
Peel the melon slices thinly with a pointed knife. The watermelon slices should be about 14 cm Ø, the other two melon slices at least 10 cm Ø. Use a round cookie cutter (approx. 5 cm Ø) to cut out the seeds from the honeydew melon and Charentais melon.
Dab melons dry with kitchen paper.
Mix the sugar and cream setting agent. Whip 250 g cream until stiff, allow the sugar mix to trickle in. Stir mascarpone until smooth. Fold in cream. Approx. 1⁄4 Remove the cream, stir in the almond mix and set aside.
Place 1 watermelon slice on a cake plate and spread approx. 1 cm thick with mascarpone cream. Place the Charentais melon on top. Spread again with some cream (approx. 0.7 cm thick). Place the honeydew melon on top.
Fill the resulting hollow with almond cream. Again spread mascarpone cream approx. 1 cm thick. Place a slice of watermelon on top.
Whip 250 g cream until stiff. Fold into the remaining mascarpone cream. Spread the cake with the mascarpone cream. Sprinkle the top and bottom edges of the cake with the remaining pistachios. Chill for about 1 hour. Rinse and sort blueberries and drain well.
Spread on the cake just before serving. Dust with icing sugar.