For the dough put flour and 1 pinch of salt in a mixing bowl. Grate butter on a coarse grater directly into the flour. Mix briefly with a wooden spoon. Stir 50 g sour cream, lemon juice and 4 tbsp. ice-cold water until smooth, add to the flour mixture. Knead first with a spoon and then briefly with your hands until all ingredients are combined.
Roll out the dough on a little flour and make it round (about 30 cm Ø). Place in a greased tart tin with a raised base (26 cm Ø), dusted with flour. Press the edge of the dough and prick the base several times with a fork. Chill for about 1 hour.
Wash and clean the mushrooms if necessary. Clean and wash spring onions and cut into fine rings.
Heat the oil in a large frying pan. Fry the diced ham until crispy. Remove. Fry the mushrooms in the frying fat until all the liquid has evaporated. Fry the spring onions briefly. Season with salt and pepper. Add the diced ham again.
For the glaze, mix eggs and 150 g of sour cream. Season with pepper and some salt.
Spread the dough base with breadcrumbs. Spread the mushroom mixture on top and pour the egg icing over it. Bake the tart in a preheated oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer) for approx. 40 minutes.