Mix the linseed and water, leave to absorb for approx. 30 mins.
Mix the flour, cornstarch, baking powder and salt. Add the butter, rub roughly by hand. Add the sour single cream and soaked linseed, knead to form a dough. Shape the dough into a ball, flatten a little, cover and leave to chill for approx. 30 mins.
Roll out the dough (approx. 4 mm thick) between two sheets of baking paper, chill for approx. 15 mins. Cut out circles (each approx. 10 cm in diameter), place in the prepared tins, prick firmly with a fork, chill for a further 15 mins.
Heat the oil in a non-stick frying pan. Fry the mushrooms in batches for approx. 3 mins. until golden brown, season, allow to cool slightly. Mix together the sour single cream, cottage cheese and egg. Stir in the parsley, season. Pour the filling into the tarts, top with the mushrooms.
Approx. 30 mins. in the lower half of an oven preheated to 200°C. Remove from the oven, allow to cool slightly, remove from the tart tins and serve lukewarm.