Remove the mushroom shafts and place them in a gratin dish with olive oil.
Coarsely chop the removed stem and bacon.
Insert the bacon and stem into the dent in the mushroom and sprinkle with chopped garlic and black pepper.

Drizzle with olive oil, cover with aluminium foil and bake in the oven at 240°C-250°C until the mushrooms are tender, remove the aluminium foil and brown slightly.