Preheat oven to 425˚F.
Wipe the mushrooms with a damp paper towel, or rinse and drain the mushrooms. Twist and remove the stems from the caps. Place the caps on a baking sheet and set them aside. Finely chop the stems.
Melt the butter in a skillet over medium heat. Add the onions, garlic, and chopped mushroom stems. Stir and cook until tender, about 5 minutes.
Add the bread crumbs, 1/2 cup of cheese, pepper, and salt. Stir to combine and cook until the cheese softens. Turn off the heat and let the stuffing mixture cool.
Spoon the stuffing into the mushroom caps when it is cool enough to handle. Pack in the filling lightly, and top with the remaining cheddar cheese.
Bake in a preheated oven for 15 minutes until the mushrooms have softened, and the cheese melts. Serve warm.
Yield: 15 to 20 stuffed mushrooms