Grease a tarte- or quiche dish with a removable bottom (approx. 22 cm Ø) and dust with a little flour. Separate 1 egg for the dough. Put the egg white in a cool place. Knead 200 g flour, approx. 1/2 tsp salt, butter in pieces, egg yolk and 2 tbsp ice-cold water first with the dough hooks of the hand mixer, then with your hands to a smooth dough.
Roll out the dough on a little flour until round (approx. 25 cm Ø). Place in the tart tin and press down at the edge. Prick the base several times. Cover the tin with foil and chill for about 30 minutes.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Pre-bake the dough for about 20 minutes.
In the meantime peel the onions and cut them into thin strips. Wash the herbs and shake dry. Chop rosemary finely. Wash and halve the tomatoes. Mix goat cheese, 1 egg and 1 egg white. Season with salt and pepper.
Remove the base from the oven and spread the cheese cream on top. Spread tomatoes with the cut surface facing upwards, onions, rosemary and sage leaves on top. Season with sea salt and pepper.
Bake the tomato cake in the oven at the same temperature on the lowest shelf for 1-1 1/4 hours. Remove the cake from the tin. Tastes hot or cold.