Knead all the ingredients for the dough. Wrap the dough in foil and let it rest in the refrigerator for an hour.
In the meantime, peel the onion and cut it into very fine slices and sweat in the butter until translucent. Dust with the sugar and let it caramelize. Add the sauerkraut and bay leaf and deglaze with the prosecco. Season with salt and pepper and simmer over medium heat for about 30 minutes.
Grease four mini tart molds with butter. Quarter the dough. Roll out each quarter on the floured work surface and line the molds with it. Spread the sauerkraut on the dough.
For the topping, stir all ingredients until smooth and season with salt and pepper. Pour evenly over the sauerkraut.
Halve the grapes, remove the seeds if necessary. Place on the mini tarts with the cut surface facing up.
Bake at a preheated 180 ° fan for approx. 20 minutes. Let cool a little and remove from the molds.
I like to serve them with cheese dumplings. To do this, cut a slit in 12 grapes. This will remove the seeds and some pulp. Put a small piece of cheese (I used Toma Morbido) in each and bake for 5 minutes at 180 °.