Preheat the oven to 200?F.
Pour the oil into a large roasting pan and add the mushrooms, spreading them out evenly and pressing them down gently. Add the bouquet garni and submerge in the oil.
Separate the split garlic head into cloves and submerge them in the oil-mushroom mixture. Season with salt and pepper. Cover the pan with a sheet of heavy-duty aluminum foil and slow-bake for 2 hours.
Drain the mushrooms, reserving the oil.
Yield: about 1 pound