Chop the chocolate and melt with the butter over a warm water bath. Roast the almonds in a pan without fat for about 5 minutes, turning constantly. Remove the almonds from the pan, chop finely and stir into the chocolate mixture.
Spread the bottom of a springform pan (26 cm Ø) with oil. Spread the crumb mixture evenly and press down. Place the base in a cool place for 15-20 minutes.
Meanwhile, wash the lemons hot, grate them dry, finely grate the peel and squeeze the juice (should give 50 ml juice). Soak gelatine in cold water. Separate the eggs. 6 egg yolks,
Stir the lemon zest and juice into the egg cream. Squeeze out the gelatine and dissolve carefully in a small pot. Stir 3-4 tbsp. egg mixture into the dissolved gelatine. Then fold the gelatine mixture into the remaining egg mixture.
Whip cream and 3 egg whites separately until stiff and fold into the lemon cream with a whisk. Spread the cream on the base and spread it loosely with a spoon (so that small hollows are created).
Chill the cake for about 4 hours.
Wash and clean the rhubarb and cut it into diagonal pieces about 3 cm long. Boil up the nectar and 1 tablespoon of sugar. Stir starch and 1 tablespoon of water until smooth. Stir into the boiling juice, bring to the boil and simmer for about 1 minute while stirring.
Add rhubarb, bring to the boil and simmer for 2-3 minutes. Remove from the heat and pour into a bowl. Just before serving, spread the rhubarb compote on the cake.