Fry the bacon in a pan without fat. Steam the shallot in butter until translucent, add the mushrooms and sauté. Dust with flour and mix in the bacon cubes. Top up with bouillon and bring to the boil. After 10 minutes, pour in the cream and season with salt and pepper. Let the soup cool down and add the chives.
Fill the soup into 4 soup cups (ovenproof). Cut out the defrosted puff pastry a little larger than the cup dia. Mix the egg yolk and cream and brush the edges of the cup with it. Place the lid on it and press down firmly. Brush the lid with egg yolk and let rest for 20 minutes.