Wash and dry the mushrooms. Carefully remove the stems using a pinch-and-twist motion. Collect the stems and set the mushroom caps aside.
In a food processor, finely chop the mushroom stems and add to a large bowl. Next add the garlic to the processor and chop fine. Scrape the garlic into the bowl. Then add the bread slices to the processor, and chop until you have soft breadcrumbs. Add to the large bowl.
Add the cheese, egg, salt, pepper, and parsley to the bowl and stir until everything is combined. The mixture should be moist and just a little sticky. If not, you may want to add a tablespoon or two of water, just to help keep it together as you stuff the mushrooms.
Next, stuff each mushroom cap with the filling; you'll want to pack it fairly tight so that it will stick.
At this point you can place the mushrooms in an airtight container overnight, or just take them right to the oven.
Bake on a lightly greased baking sheet at 350F for 25-30 minutes, until the mushrooms have softened and the filling is nicely browned. Cool for just a couple of minutes and serve warm.