Preheat the oven to 350°F.

Wipe the mushrooms with a damp cloth or mushroom brush to remove any dirt or debris. Remove the stems and scrape out gills. Finely chop the mushroom stems.

In a medium skillet over medium heat, sauté the onion, garlic, and mushroom stems in olive oil until soft and tender.

Remove the pan from heat and add the Ritz cracker crumbs, cheese, melted butter, and black pepper. Mix well. If the mixture is too dry, add more olive oil or butter.

Stuff each mushroom with the stuffing mixture and place them in a baking dish in a single layer.

Cover with foil and bake for 20 minutes. Remove the foil and bake for 10 more minutes until golden brown on top.