Carefully remove the stems from the mushrooms. Set the caps aside and chop the stems finely.

Melt the butter in a wide skillet over medium heat. Add the onion and garlic and gently cook until softened, about 2-3 minutes. Add the chopped mushroom stems and saut? until softened, about 5 minutes.

Add the breadcrumbs and Italian seasoning to the mushrooms in the pan and stir well to coat everything with butter. If it seems dry, add a little more butter. Cook for another 3-4 minutes, take off the heat and stir in the parmesan, parsley, and salt and pepper (if needed). Then set aside to cool.

Preheat the oven to 350?F. Evenly distribute the filling between the mushroom caps, pushing it slightly down as you fill them. Place the mushrooms on a baking sheet lined with nonstick silicone mats and bake until the top is browned, about 20-25 minutes.

Serve hot or warm, with a bit of lemon juice if you like.