For the dough, grate the flour, butter and salt into a crumbly mass. Add water and combine to form a smooth dough, but do not knead. Cover and chill for 30 minutes.
Roll out the dough around 3 mm thick on a little flour. Line the baking tray lined with baking paper with it, prick the pastry base with a fork.
Mix the milk and flour for the topping, bring to the boil while stirring, simmer for 5 minutes, allow to cool slightly.
Mix in the cheese, yoghurt and onions, season. Spread the whole thing on the dough base.
Bake for 45 minutes in the lower third of the oven preheated to 200 ° C.
Alternatively, a ready-made cake or puff pastry can be used.