Separate eggs. Beat the egg whites, 4 tablespoons of water and 1 pinch of salt with the whisk of the hand mixer until stiff, adding 100 g of sugar at the end. Stir in the egg yolks one after the other. Mix flour, starch and baking powder, sieve over the egg mixture and fold in. Line a baking tray (approx. 30 x 38 cm) with baking paper. Add the mixture and spread evenly. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 10 minutes
Remove the baking tray from the oven, turn the sponge cake onto a tea towel sprinkled with sugar, carefully remove the baking paper. Roll up from one long side with the tea towel, let it cool down for about 1 hour
Soak gelatine in cold water. Mix yoghurt, 4 tablespoons sugar, lemon peel and 1-2 tablespoons juice. Squeeze the gelatine, dissolve, stir in 3 tbsp. yoghurt, then stir into the remaining yoghurt, chill for approx. 10 minutes until the cream begins to gel
Wash the untreated orange thoroughly, rub dry, finely grate the peel and set aside. Peel all oranges so that the white skin is completely removed. Remove the fillets with a sharp knife between the parting skins, cut into large pieces. Whip the cream with the whisk of the hand mixer until stiff. Fold cream first, then orange fillets into the yoghurt
Unroll the sponge cake, spread the cream on top and spread evenly, chill for about 10 minutes. Roll up the sponge cake from the long side with the help of the cloth, refrigerate for about 2 hours. Mix icing sugar with 1-2 tbsp. lemon juice to a thick icing. Pour the icing into a freezer bag, cut off a small corner. Spray thin lines on the roll, dust with icing sugar, chill for about 30 minutes
waiting time approx. 1 3/4 hours