Prep. Preheat the oven to 375F. Spray the inside of a 9 by 13-inch casserole dish with olive oil cooking spray.
Wash and chop the mushrooms. Use a dry or slightly damp cloth to gently brush off any dirt on the mushrooms. Remove the stems from the mushrooms and set the mushroom caps aside for now. Finely chop the mushroom stems (you should get about 1 1/4 cups of chopped stems).
Make the filling. Heat the oil over medium to medium heat, then add the chopped stems and cook until softened, about 8 to 10 minutes, stirring occasionally. Add the onion and cook until the onion is softened, about 5 to 7 minutes, stirring occasionally. Add the garlic and cook 1 minute, stirring constantly.Transfer the mushroom stem/onion mixture to a bowl and allow to cool slightly. To the cooled onion mixture, add the eggs, ricotta, parmesan, dried Italian herb seasoning, salt, black pepper, and breadcrumbs. Mix to combine.
Stuff the mushrooms. Scoop or spoon the stuffing mixture into the mushroom caps (gently pressing it down into the mushroom cavity), and arrange the filled mushrooms in an even layer in the prepared baking dish. Sprinkle the mozzarella evenly on top.
Bake. Cover the dish with foil and bake for 30 minutes, and then remove the foil. Preheat the broiler. Broil a couple minutes to brown the cheese (watch carefully so they don’t burn).
Enjoy. Sprinkle the fresh parsley on top and serve.
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Notes
Recipe Yield: Depending on how big your button mushrooms are, you will get a different number of stuffed mushrooms. I usually get about 30 to 35 stuffed mushrooms from 24 ounces of button mushrooms.
Serving Size: In addition to the yield, the serving size will also vary based on how big your mushrooms are. In general, this recipe makes about 8 (appetizer or first course-sized) servings. As an example, if you use 32 mushrooms, you'll get 8 servings with 4 stuffed mushrooms per serving.
Storage: Store leftover stuffed mushrooms covered in the fridge for up to 5 days.
Reheating: You can reheat leftover stuffed mushrooms in an oven-safe pan covered with foil in a 350F oven until warm, about 10 to 15 minutes.