Preheat the oven to 400 degrees F. Grease
a 9-by-13-inch baking dish
with olive oil.
In a large bowl, mix together the breadcrumbs, Parmesan
, garlic, parsley and 3 tablespoons of the oil. Sprinkle with salt and pepper and stir to combine.
Remove the stems from the mushrooms and finely chop
them, then stir them into the breadcrumb mixture. Use a teaspoon to hollow out the mushrooms (remove and discard the brown gills), then fill each mushroom cap with 2 heaping tablespoons of the stuffing
. Place the mushrooms in the prepared baking dish and drizzle
the remaining 3 tablespoons of oil over the tops. Bake until the stuffing in the mushrooms is bubbling and the tops are well browned, about 20 minutes. Transfer to a platter and serve.