Slice the mushroom stalks from the caps. Place the mushroom caps on a baking sheets, gills facing up. Finely chop the mushroom stalks.
In a bowl, combine the mushroom stalks, mint, parsley, Parmesan, garlic, and bread crumbs and mix thoroughly. Using a spoon, divide the mixture evenly among the 6 mushroom caps, gently packing the mixture down with the back of the spoon.
Drizzle a little olive oil over each mushroom, then bake for 15 to 25 minutes, until the surface of the stuffing is golden. Serve warm.
Stuffed Mushroom Variations
Blue Cheese-Stuffed MushroomsOmit the mint and Parmesan and reduce the bread crumbs to 2 tablespoons. Add 2 3/4 ounces crumbled blue cheese, such as Gorgonzola, in place of the Parmesan.Bite-Size Stuffed MushroomsSubstitute 15 or so button or cremini mushrooms for the portobello mushrooms. You will not need to use all the chopped mushroom stems.