Pre-steam mushrooms with bacon, pepper and salt, hard-boil eggs and then cut them in half, take out hard yolks and crumble them into the frying pan.
Stir everything together.
Grease gratin dish, fill egg white halves with farce and place in dish. Spread the rest evenly over the top. Sprinkle with grated Parmesan cheese, spread butter flakes evenly on top and bake in broiler or possibly oven.