Move your oven rack to the top position. Preheat oven to 450⁰F.
Wipe mushrooms of any dirt with a dry paper towel. Remove stems, so that the caps create a small bowl. Place the caps, upside down on an aluminum foil lined baking sheet. Dice the stems and set aside.
Place bread slices in a food processor and pulse until you get breadcrumbs. Add onions, parsley, garlic, and salt. Pulse to combine. Add cheese, and pulse again. Add vegetable broth, a little at a time, until the stuffing is moist and sticks together.
Fill each mushroom cap with ½ tsp of filling.
Bake caps 10 -12 minutes, until stuffing begins to crisp and mushrooms begin to soften.
Cool slightly before serving.