Brush the bottom of a springform pan (26 cm Ø) well with oil. Roast the almonds in a pan without fat. Remove and chop finely. Chop the chocolate and melt with the butter in a hot water bath.
Stir the almonds into the chocolate mixture. Pour the mixture into the mould and press it evenly into a base. Chill for 15-20 minutes.
Soak gelatine in cold water for at least 5 minutes. Wash the lemons hot, dry and finely grate the peel. Squeeze the lemons. Separate whole eggs. Beat egg whites until stiff, chill.
Whip cream until stiff, also refrigerate.
Mix all egg yolks, 135 g sugar and vanilla sugar with the whisks of the mixer for about 5 minutes until creamy. Stir in lemon peel and lemon juice. Squeeze the gelatine, dissolve carefully in a small pot at very low heat.
Stir 3-4 tbsp. lemon cream into the gelatine. Then stir everything into the rest of the cream. Fold cream first, then beaten egg white into the lemon cream. Spread loosely with a spoon on the chocolate-almond base.
Chill the cake for at least 4 hours.
In the meantime clean and wash the rhubarb and cut it into pieces of about 3 cm length. Boil up cherry nectar and 1 tablespoon sugar. Stir starch and 1 tbsp. water until smooth. Stir into the boiling juice, bring to the boil and simmer for about 1 minute.
Add rhubarb, bring to the boil again and simmer for 2-3 minutes. Remove from the heat and allow to cool. Just before serving, spread the rhubarb compote on the cake.