For the filling, rub mushrooms with kitchen paper or clean them with a brush and cut them into slices. Clean the leek, halve lengthwise, wash and cut into fine cubes. Wash, quarter, core and dice the pear. Cut bacon into small cubes.
Heat the oil in a pan. Fry the bacon in it for approx. 5 minutes until crispy, take it out. Fry the mushrooms in the remaining fat for about 3 minutes. Add pear and leek and fry for about 3 more minutes. Stir in bacon, season with salt and pepper.
Cut baking paper into six squares (each approx. 15 x 15 cm) and line six wells of a muffin tray (twelve wells) with it (it is best to press paper into the wells with a suitable glass). Roll out the puff pastry. Cut out 6 circles (approx. 11 cm Ø each) and place them on the bottom of the muffin tray. Place the rest of the dough on top of each other and roll out on a little flour. Cut out small stars, stripes or pâté lids (approx. 7 cm Ø each).
Preheat oven (electric cooker: 220 °C/circulating air: 200 °C/gas: see manufacturer). Separate the egg. Beat the egg white with a fork and stir into the mushroom filling. Briefly fold in 1 tbsp. flour. Spread into the puff pastry bases. Place dough stars, strips or lids on the pies. Whisk egg yolk and milk, brush the dough with it and bake in a hot oven for about 15 minutes.