Cut the vanilla pod in half lengthwise and scrape out the vanilla pulp. Put curd cheese and mascarpone in a bowl, stir until smooth. Mix eggs, 250 g sugar, salt, lemon juice and zest one after the other with the whisks of the hand mixer.
Grease the bottom of a rectangular springform pan (34 cm x 24 cm; with 7 cm high rim) and dust with flour. Pour the mixture into the mould and smooth it down. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 50-60 minutes on the bottom shelf.
In the meantime, sort and wash the blueberries. Put 200 g blueberries, 100 g sugar and 200 ml water in a pot. Bring to the boil while stirring and simmer for 2-3 minutes. Remove from the heat and pass through a sieve into a pot.
Mix the starch with 3 tablespoons of water until smooth. Bring blueberry juice to the boil again, thicken with starch and simmer for another 1-2 minutes. Stir in the rest of the blueberries, let cool down. Remove the cake, remove from the edge of the springform pan and let it cool in the pan on a cake rack.
Remove the edge of the springform pan after about 30 minutes. Let the cake cool down. Cut the cake into pieces and serve with blueberry porridge. Decorate with lemon balm.