I got this recipe from Cooks.com, with a few slight variations based on ingredients I already had on hand.
2 packages of large mushrooms (washed & de-stemmed)
2 cups of cheese and garlic croûtons, crushed into a crumbly consistency (original recipe called for seasoned breadcrumbs)
2 tablespoons Peccorino cheese (original recipe called for Romano cheese but I already had the Peccorino so I used that instead. The recipe also suggested using 3 tablespoons of cheese but I chose to cut the amount back a bit.)
2 tablespoons jarred tomato sauce
salt and pepper to taste
Wash and remove the stems of all the mushrooms. Place on a paper towel for a few minutes and blot dry. Reserve 1/2 the stems and dice for use later in the recipe.
Mix all other ingredients together adding olive oil to create a crumbly texture. (for extra flavor you can save the mushroom stems and dice them and add them to the mixture as well).
Preheat oven to 375 degrees. Stuff the mushrooms using a regular sized spoon and place about 1 inch apart on a greased baking sheet (I used olive oil cooking spray rather than butter). Grate some cheese over the top of the mushrooms just before putting them in the oven.
Bake at 375 for about 15-20 minutes or until the stuffing looks golden and crispy.
Notes: Delicious! I made around 25 mushrooms with this recipe but had a lot of the filling leftover, so you could probably cut back on the croutons a bit and still have plenty.