Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Grease a fat pan (deep tray; approx. 32 x 39 cm) and dust with flour. Wash organic lemons hot, dry and grate the peel. Squeeze all lemons.
Separate 6 eggs. Cream 350 g butter, 350 g sugar and lemon zest with the whisk of the mixer. Stir in 3 eggs and egg yolk one after the other. Mix 275 g flour, 100 g starch and baking powder. Alternately stir 150 ml lemon juice into the butter and egg cream. Smooth the dough on the fat pan. Bake in a hot oven for about 20 minutes, then let it cool down.
For the buttercream mix egg white and 200 g sugar. Whisk warm (approx. 60 °C) in a hot water bath with the whisks of the mixer. (Finger test: Dip fingertip a little into the cream. It must feel warm, almost hot). Remove the beaten egg whites from the water bath and continue beating for approx. 10 minutes until cooled down. Gradually stir in 250 g of softened butter in flakes (initially the mixture is slightly crumbling, just keep stirring). Chill the butter cream for about 30 minutes.
Meanwhile peel banana and kiwis. Peel the orange. Peel the pineapple, remove the stalk. Cut the fruit into slices or pieces. Sort raspberries and blackberries, possibly wash and halve or slice. Wash the currants and grapes, pluck them from the stalks. Cut the grapes in half. Wash and stone the apricots and cut them into slices.
Boil up the remaining lemon juice with 30 g sugar in a small pot. Stir 1 tbsp. starch and 2 tbsp. water until smooth. Pour into the lemon water while stirring, bring to the boil and simmer for about 1 minute. Spread the buttercream on the cake until smooth. Cover with prepared fruit in a checkerboard pattern and drizzle with lemon sauce.