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 sauté 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.