For the mushroom pie, heat oil in a pan, add diced bacon and finely diced onion and fry slowly. When the bacon and onions have color, add the chopped mushrooms, continue frying until the mushrooms also have some color and no longer give off water.
Put everything aside and in another pan fry the mince. When the mince is well browned and crumbly, mix in the mushroom mixture and let this mixture cool on a plate. Season to taste.
Rinse a pie dish with ice-cold water, roll out the puff pastry and cut it in half. Separate a narrow edge from one half for decoration.
Place the larger half of the pastry in the pie dish, stretch the pastry slightly and raise a rim of about 2 cm. Pour in the cooled mushroom mixture.
Now, rather centrally, cut a figure out of the second half of the dough and carefully place the dough lid over the filling. Push the edge down a little so that it can join the bottom dough.
Pour the beaten egg through the dough hole onto the filling, brush with the rest of the pastry. Top with mushrooms cut out and brush them as well. Sprinkle coarse grains of salt on top and bake the mushroom pie in the oven at about 170 degrees hot air for about 30 minutes on medium high.