Heat oven to 400°F. Spray a rimmed baking sheet with cooking spray or brush with oil and set aside.
Rinse mushrooms under cool water and thoroughly dry with a paper towel or clean dish towel. Slice the very ends off of the mushroom stems and discard, then gently twist the stems back and forth to remove them from the caps. Dice the stems finely.
Place each mushroom cap cavity-side up on the prepared baking sheet.
Add 2 Tablespoons of the olive oil to a medium pan and set on the stove over medium heat. Once heated, add the diced mushroom stems and sauté for 6 minutes. Add the garlic and cook for 1 additional minute.
Transfer the sautéed mushroom stems and garlic to a medium mixing bowl. Add the remaining 6 Tablespoons of olive oil, breadcrumbs, Pecorino Romano (or Parmesan), parsley, and pepper. Stir until everything is evenly combined - it should be moist.
Stuff each mushroom cap with the filling. Bake for 25 minutes, or until golden brown and thoroughly heated throughout.
