<Preparations> Chop the ingredients into pieces. Cut the onion into thin slices. Parboil the spinach, and let it cool. It's ok to use a microwave. Bring out the puff pastry to room temperature.
Add the eggs, milk, Parmesan cheese, salt and pepper into a bowl. Mix well and add the spinach.
Stir-fry the bacon, onion, and shimeji mushrooms in a frying pan. When cooled, add into Step 2 bowl, and mix well.
Stretch out the puff pastry, and stack them together in a pan. Punch holes on the bottom part of the puff pastry with a fork. Pour in the mixture.
Bake in the oven for about 25 minutes at 200℃ (400℉), and it's finished.